Hi Charles and welcome. Collectors like the ability to select sequential serial numbers, others match special events such as birth days, anniversaries, etc. Lots of reasons actually. Cheers and stay safe!

Different reasons but mainly for a personal touch. That serial number one might pick could have meaning to them. A Birthday, anniversary, In memory of date, the list can go on.

Mainly for collectors.

Charles, most people wouldn't want to. For others, certain numbers may have a special meaning. On this firearm, 061374 is available, that could be someone's birth date, June 13, 1974. . And for limited edition or rare firearms, people may want the lowest serial number possible for value purposes.
Stay safe & have a great day!