Just a small clarification re the a used
between a verb and a direct object like: "llamar a la policia."
this is called the personal a and is used when the direct object is a person or possibly a pet. Although in the above example it is correct to translate as "to", typically it is ignored when translating to English eg
No conozco a tu ...
They are related but are not the same:
Acudir entails its hypernym ir, but moreover you go because you know in advance what happens or is gonna happen there: maybe it's an appointment, a public/private service, party or public event.
Asistir prototypical meaning is closer to witness or attend. The verb is frequently used for attending public events or ...
Acudir a, acudieran a => x come to sb (for complaining, for support, for help [go to the police/press])
6. intr. Recurrir a alguien o valerse de él.
Acudir/ir a la fiesta => to go to the party, to attend the party, come to the party, to show up at the party (aparecer, llegar, venir, ir).
2. intr. Ir o asistir con frecuencia a alguna ...
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 ...
In Spanish, you say: ir a un lugar.
Ergo, it's logical to say: ¿A donde vamos? It is also said: ¿Donde vamos? just like English.
However, in English, though we say go to a place, there is no need for the preposition to in the sentence.
Also "asistir" can mean "to help" someone.
Eg.: Asistí a una mujer con su parto | I helped a woman in labor.
'acudierien en masa' - "they went in mass" -> means "if they go in big numbers". Acudieren in this case means "in case of". They went in mass -> literally: "Fueron/Acudieron/Asistieron en ...