Some verbs cause a noun to automatically take the Ablative case
These verbs typically mean "keep away from, free from, deprive, lack" etc (thelatinlibrary)
Some examples of these verbs include: utor, fruor, fungor, potior, carere, egere
Prepositions a, ab, ex can all be used
Ex: ab urbe proficisci- to set out from the City (thelatinlibrary)
City is in the ablative because the verb proficisci forces it. The sentence is forced to say "to set out from the city."
Ex: castris egressi- leaving the camp (thelatinlibrary)
This sentence does not use a preposition, because it is not always needed. The verb egressi forces castris to take the ablative form. Since castris is in the ablative, this sentence can me more literarally translated as "leaving from the camp"