Everybody has their own way to approaching ConvertKit.
A common requirement is "on boarding" - where every new subscriber should get the same series of emails welcoming them to your list. That's where sequence shines - sending the same emails out to different people at different times. (Sequences come with a paid plan)
Once you have them on the list, how do you tell them things? That's where broadcast comes in. You can send an email (like a periodic newsletter) to all or part of a list based on a series of logical conditions. (Broadcasts are available for free accounts)
I can't tell you whether you need to upgrade or not. Here's the pricing page that shows what each tier does: https://convertkit.com/pricing72