The "A" between 2 verbs

[Sang]

I'm still very confused about the use of the "a" between the 2 verbs. Hopefully, you can help me. For example:
se prohibe fumar = it's prohibit to smoke
se dedican a defender .... = they are dedicated to defending...

Why is there an "a" on the second sentence and not on top? it's necessary?

[poli]

Some Spanish verbs are generally followed by a. It's a rule that has some logic to it, but it's best to memorize them.
Here's a list:
https://www.learnspanishnow.online/spanish-verbs-followed-by-preposition-a/

[AngelicaDeAlquezar]

I agree with Poli, most of the prepositions that follow a verb have to be memorized, but they will just come naturally with practice.
The prepositions that follow the verbs depend on the kind of action the verbs express, and the meaning of the preposition.

- Voy a la escuela.
- Voy a estudiar.
-> The verb "ir" gives an idea of a destination, which is expressed by the preposition "a".
- Quiero un helado.
- Quiero comer un helado.
-> The nature of the verb "querer" doesn't need any preposition to introduce the direct object.
- Vine a México.
- Vine a ver una película.
-> The verb "venir" needs the preposition "a" to express a destination or a purpose.