The B-flat clarinet is the standard instrument for beginners, school bands, jazz and most popular repertoire, and is what's meant by default when someone says simply "clarinet." The A clarinet, pitched a half-step lower, exists specifically because certain classical and orchestral works (notably much of the Romantic-era repertoire) are written in keys that are considerably easier to read and finger on A clarinet than transposed onto B-flat. A beginner has no practical need for an A clarinet, and it typically only becomes relevant once a player is performing in an orchestral or serious classical ensemble setting that specifically calls for it, at which point it's bought as a second instrument alongside, not instead of, the B-flat.