What is meant by sahoor in the hadiths that speak of its virtue is the sahoor of those who fast in particular

Praise be to Allah

Sahoor refers to the food or drink that is consumed by a person at the end
of the night. It is called sahoor because it is consumed at the time of
sahr which is the end of the night. 

Lisaan al-‘Arab
(4/351) 

There are a number of hadiths that speak of the virtue of sahoor, such as
that in which the Prophet (blessings and peace of Allah be upon him) said:
“Eat sahoor, for in sahoor there is blessing.” Narrated by al-Bukhaari
(1923) and Muslim (1095). And he (blessings and peace of Allah be upon him)
said: “The difference between our fasting and the fasting of the People of
the Book is the meal before dawn (sahoor).” Narrated by Muslim (1096). And
he (blessings and peace of Allah be upon him) said: “Verily Allah, may He be
exalted, and His angels send blessings upon those who eat sahoor.” Narrated
by Ahmad (11086); classed as saheeh by the commentators on al-Musnad.
Classed as hasan by al-Albaani in as-Saheehah (1654) 

What is meant by sahoor in these hadiths is the food eaten before dawn by
the one who is going to fast in particular, because that food gives the
fasting person strength for the act of worship that is fasting, and makes it
easier for him. Moreover it is the sin that differentiates between our fast
– us Muslims – and the fast of the People of the Book. This is understood
from the comments of the scholars about the reason why sahoor is regarded as
a blessing. 

An-Nawawi (may Allah have mercy on him) said: 

The scholars are unanimously agreed that it (sahoor) is mustahabb but is not
obligatory. As for the barakah that is in it, it is obvious, because it
gives one strength and energy to fast, and because of it one wants to fast
more, because it reduces the hardship involved for the one who eats sahoor.
This is the correct view concerning its significance.

End quote from Sharh an-Nawawi ‘ala Muslim (7/206) 

Al-Mannaawi (may Allah have mercy on him) said concerning the words of the
Prophet (blessings and peace of Allah be upon him), “Verily Allah, may He be
exalted, and His angels send blessings upon those who eat sahoor”: That is,
those who eat sahoor with the aim of strengthening themselves for the fast,
because of what it does of calming the desires of the stomach and private
part, which leads to purification of the heart and dominance of the
spiritual over the physical, which brings one closer to one’s Lord, may He
be exalted. Therefore sahoor is very highly recommended. 

End quote from Fayd al-Qadeer (2/270) 

It
says in al-Mawsoo‘ah al-Fiqhiyyah (24/270): 

Sahoor is Sunnah for the one who fasts. Ibn al-Mundhir narrated that there
was consensus on its being recommended. End quote. 

The scholars of the Standing Committee for Issuing Fatwas said: 

What is prescribed for the one who fasts is to eat sahoor before dawn
breaks, because that gives one strength for fasting. 

End quote from Fataawa al-Lajnah ad-Daa’imah (9/26) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

“Verily in sahoor there is barakah (blessing).” Where is the barakah? All of
it is barakah, for it is an act of worship, following the example of the
Messenger, differing from the people of Hell, helping oneself to fast, and
giving one’s nafs its rights, because it will be followed by a time of
self-restraint, so it gets its share of food and drink so as to gain
strength for carrying out an act of worship; it is the precursor to the
fast. 

End quote from Liqa’ al-Baab al-Maftooh (7/147) 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

The barakah in sahoor is attained in several ways: it is following the
Sunnah, differing from the People of the Book, giving oneself strength to do
an act of worship, increases one’s energy, wards off the bad attitude that
may stem from hunger, is a means of giving charity to one who asks for it at
that time or joins a person when eating, it enables one to remember Allah
and call upon Him at a time when the response is hoped for, and it helps one
to form the intention of fasting, for one who forgot to do so before going
to sleep. 

End quote from Fath al-Baari (4/140) 

Some of these reasons mentioned by al-Haafiz Ibn Hajar (may Allah have mercy
on him) are not only applicable to the one who fasts, but they are connected
to the intention of fasting, for the intention of fasting is the basis,
which is followed by these benefits of eating sahoor. 

The scholars are unanimously agreed that sahoor is mustahabb for one who is
fasting, but we do not know of anyone among the scholars who said that it is
mustahabb for one who is not fasting. If sahoor was mustahabb both for one
who is fasting and one who is not, the Prophet (blessings and peace of Allah
be upon him) would have always eaten sahoor, but he (blessings and peace of
Allah be upon him) did not do that, which indicates that sahoor is only
mustahabb for the one who wants to fast. 

And Allah knows best.