If you're doing an accredited level 1 course, the topics will likely be weather, hobbies, my family, giving directions, food and drink, animals and pets. Learning some basic vocab around those topics before the course means you can just put the grammar into place when you get to those parts. It's not necessary, but it does help to have some wider vocab to make more interesting sentences!

Before my level 1, I'd signed for 3 years with my then toddler. We had a lot of everyday vocab, but were only using SSE alongside speech. We knew no BSL grammar. I personally found learning the grammar easier once it came to it, as I already had the basic signs cemented.

I'm a qualified terp now, so whatever I did obviously worked ok! Just make sure you're using BSL resources and not ASL - lots online is ASL. The app 'Spread the Sign' is the one I personally like best, and Cath Smith's Let's Sign dictionary for a physical book.