Years ago, car weren’t as mechanically robust and weren’t as flexible at dealing with the many different types of driving styles. As an example, in a modern car you can easily use the foot brake to slow from around 60 mph, in 5th gear all the way down to 10 mph before depressing the clutch pedal and block gear changing directly into 1st gear from 5th. Do this in an older car and you’ll likely stall the engine.
Also, modern car brakes have become more robust and efficient at doing the job. As a means to slow the vehicle, block gear changing puts the emphasis more on the car’s brakes and less on gear changing (slowing the car by use of gears is called engine braking).
Years ago, driving instructors would teach learners to slow the vehicle by changing down gears sequentially to reduce the burden on the brakes, which with heavy use could result in brake fade (see what is sequential gear changing).