Normally you'd buy the final product as well including
anything specific like 'Steam key', 'Soundtrack' or
'all future episodes'.
Alphas need some extra taking care with as often
they're quite raw. Some to the point of being not
much more than the engine. Basically an alpha is
often what you'll find at Kickstarter and such if
there's anything at all playable.
Betas you could very well just consider 'open'
betas. The pre-release version that is going to
Quality Assurance for bug testing, balancing,
stress-testing and such. QA is expensive so it's
not an option for lots of devs, hence the discount.
Hope this helps