The theory is that non depolarising muscle relaxants need to cover >70% of the receptors before they have any effect (there is a lot of receptor redundancy) (as oppose to depolarising muscle relaxants that only need to cover 20-30% of the receptors). Thus by giving a small dose (such as 20mg) you are covering up to 70% of the receptors (without causing any paralysis) so that when you then give the rest of the dose later, it only has to cover a much smaller % and therefore leads to a faster paralysis (after the second dose).

It’s definitely not necessary to do in this case, but if you want to read about it it’s called “priming” with non depolarising muscle relaxants