The "bundles" are their way of selling you extra stuff.
Say you top up with, say, £25.
They offer you a "bundle" giving you a similar number of minutes but including 2000 text messages for "only" £30.
That may be a great deal if you send text messages all day, but few of us do.
They may include data, but that isn't a lot of use unless your handset gives you a way of using it - normally you'd need a smartphone. They expect everyone to have one of these, but lots of older people just use a phone for making and receiving phone calls.