I can't think of a good reason to fly into Portland instead of Manchester, unless
1) you want to visit Portland (it can be a fun town, if you're into the whole artsy-college town-leftist sort of vibe),
During my 12 years in Maine, far northern and eastern ends, Portland was always at least 4 hours away, so I haven't been there much except to drive through. Plus it was always the "big city," isn't that funny? So this reason really doesn't apply to me.
2) it's cheaper for you fly there, rent a car, etc., or
3) you find a flight into Portland that better suits your schedule. According to google maps, Manchester is almost 1/2 hour closer than Portland to Lancaster.
I agree with these 2 reasons, only if 1 or both is true would I fly to Portland. However, I would also add:
4) You derive pleasure from simply being in a place you have never been (or haven't been in a long time), regardless of if there's anything you want to do or see there.
This would be a bonus for me, if 2 or 3 is also true. I once was visiting an area north of Chicago and drove across the border to Wisconsin for just this reason. If I rent a car on this trip I would like to drive up to Canada for just this reason (passengers with passports welcome but no passengers planning border activism, please - it's just not my choice of battle).