Mostly as a QoL and also to help make the priorities < 5 more useful. I can set the priorities of many background activities (such as sweeping/cooking/research) well enough to make use of the priorities > 5 (for example, life support stuff goes at 8; if I want to clean before work I set containers at 6, etc.). I never use < 5 unless I am explicitly planning (pre-painting the base as part of planning, for example).
Some activities - such as mopping and sweeping - I always set higher than default because I'm doing so to deal with an immediate need. And in other cases I would like to set some work to always be higher priority by default (like construction being higher priority than digging, or whatnot). I would probably set digging to 3 and construction to 4, for example, and leave 1-2 for explicit background work.
Anyway, setting the default priority for a new task seems a pretty easy way to leverage the existing system to be more flexible for the player. It could be as simple as remembering the last setting for the task type.
Since I am forced to :- ) : Click dig, select priority 4, create some dig jobs. Click harvest then dig again - job priority is back to 5.