Block gear changing is now the preferred method for changing down gears and on occasion, for changing up gear too (see what is block gear changing). For the vast majority of the time, for learning to drive, you’ll change up gear sequentially.
The only occasion where you might want to block change up gear is if you’re gaining a lot of speed on a dual carriageway slip road (as an example) in 3rd and when on the carriageway you may have no need for 4th gear, so block change directly into 5th.
Block gear changing is most beneficial when changing down gear. You’ll generally use the foot brake to slow the vehicle and miss out on unnecessary gears. This is not only easier than changing down gear sequentially (there’s less to do), but it also means you have both hands on the steering wheel more of the time.