Nasai

The adhan of Bilal (radi Allahu anhu) was neither for tahajjud (late night prayer), because there is no adhan for voluntary prayers (nawafil), nor was it for suhur (pre-dawn meal), because the adhan is for prayer, not for eating and drinking. Rather, it was for the Fajr prayer itself, but somewhat before its time. However, from this adhan, a person could benefit for tahajjud or suhur, just as one benefits from the Maghrib adhan for breaking the fast (iftar). In the time of the Messenger of Allah (sallallahu alayhi wa sallam), although there was not much interval between these two adhans, since this interval is not fixed, it can also be longer.

Hadith Footnote:
Vocabulary of the Hadith:
Sahur:
Food prepared for the pre-dawn meal (sahri).
Suhur:
The food of sahri,
food.

Benefits and Issues:
There are two types of dawn: false dawn (fajr kadhdhib) and true dawn (fajr sadiq).
During the false dawn, the pre-dawn meal (suhoor) is eaten, and as soon as the true dawn begins, the time for suhoor ends.
Bilal radi Allahu anhu used to give the call to prayer (adhan) during the false dawn to alert the people.
In the false dawn, at first the whiteness (light) rises vertically towards the sky, then soon after, the whiteness appears again and spreads across the horizon, and this is the true dawn (fajr sadiq).