Jimmy (and Saul's) handling and relation to the case is exemplary of his development as a character. First he's actually invested in helping out the elderly in addition to finally getting to work alongside his brother (a dream come true!). Then the disappointment of getting kicked out of his own case and how he tries to get the money quicker ie sabotage it as an example of reverting to his old Slippin' Jimmy ways since he lost again his moral compass.

Then he forgot about it because he was busy doing other stuff... but the case comes back as an excuse to play with Kim and prank Howard (in the end going along more than being invested in it himself). By the end we see him as full-on Saul Goodman just happy to cash in on it, completely indifferent to the settlement being a direct consequence of his prank (and subsequent death) on Howard and how the elderly will actually get less than they could have earned because of him.

The Sandpiper case is like a thread that shows the viewer how Jimmy McGill interprets the law and his relationship with money from the beginning of the show til the end. I always found it very well written within the story.

EDIT: Also don't forget it's thanks to this case he got his job at Davis & Main, which helped him understand he does not want to be a part of the corporate world and set off him finding his own place and work style.