To mirror your question: I've been studying the preposition “for” and how it's used, what I have found so far is:
for (fôr; fər when unstressed)
a. Used to indicate the object, aim, or purpose of an action or activity: trained for the ministry; put the house up for sale; plans to run for senator.
b. Used to indicate a destination: headed off for town.
2. Used to indicate the object of a desire, intention, or perception: had a nose for news; eager for success.
a. Used to indicate the recipient or beneficiary of an action: prepared lunch for us.
b. On behalf of: spoke for all the members.
c. In favor of: Were they for or against the proposal?
d. In place of: a substitute for eggs.
a. Used to indicate equivalence or equality: paid ten dollars for a ticket; repeated the conversation word for word.
b. Used to indicate correlation or correspondence: took two steps back for every step forward.
a. Used to indicate amount, extent, or duration: a bill for five dollars; walked for miles; stood in line for an hour.
b. Used to indicate a specific time: had an appointment for two o'clock.
c. Used to indicate a number of attempts: shot three for four from the foul line.
a. As being: take for granted; mistook me for the librarian.
b. Used to indicate an actual or implied listing or choosing: For one thing, we can't afford it.
7. As a result of; because of: jumped for joy.
8. Used to indicate appropriateness or suitability: It will be for the judge to decide.
9. Notwithstanding; despite: For all the problems, it was a valuable experience.
a. As regards; concerning: a stickler for neatness.
b. Considering the nature or usual character of: was spry for his advanced age.
c. In honor of: named for her grandmother.
Because of this; for this reason.
Sames goes for Spanish “a”, which is a preposition and very polysemous grammaticalized word (it translates as: it has many meaning and it helps create many types of structures and relations between words) — you have a good (not completely exhaustive, but reasonable) list here:
I wouldn't fixate too much on finding out all uses of that word if you are in your early stages of learning Spanish, as many of those people tend to learn organically.
That being said, you touched on the most important ones. The uses you quoted specifically are:
- ir a creates a future tense-like structure, such as English “to be going to (do something)” or “will soon do something”.
- a in structures such as llamar a comes from two places at the same time, kind of. First one is just what a verb sometimes needs: compare “to” sometimes connecting these words: I gave him the package vs I gave the package to him. The other reason (the one more important here) is that whenever you “do something” to an animate entity (mostly humans, but some socially important animals, such as dogs, cats and horses) you have to mark it with this preposition, i.e.: https://www.spanishdict.com/answers/272049/another-personal-a-question. In other words, if you fail to use preposition with animate words, you are commiting a grammatical mistake and are borderline rude by treating people as stuff.
- a can also introduce the goal of movement. Spatial relationships are the primary function of core prepositions, both logically and etymologically. Hence, vamos a la playa means going to the beach – on a tangent, a is similar to hacia here, in the same sense that to is similar to towards (but the latter pair doesn't specify reaching the destination, only the direction).