Also can anyone explain why you need a ‘ber’ in the first place, what is the difference between ‘Saya jalan’ and ‘Saya berjalan’? It wasn’t clear to me in the intro...
(ber-) + (noun) = verb
The word "jalan" is a noun, so it needs the ber- prefix to use it as a verb (in the formal way).
"Saya jalan" is informal.
The word "jalan" is used as a verb in this sentence.
As you can see in the dictionary, it's only used as a verb in the informal way.
That's the reason the ber- prefix is used here.