Yes, Doug Baldwin is primarily a backup for Seattle. Yes, Doug Baldwin was an undrafted free agent. No, Doug Baldwin is not a #1 WR. Despite all those caveats, Doug Baldwin still produced better stats than any Panthers WR did last year despite not even being a full-time starter. That is not all. While Baldwin's productivity dipped in 2012 because of a lack of playing time, a new QB and an offense that emphasized the zone read and the running game, in 2011 he had similar numbers to 2013, statistically one of the best rookie WRs in the NFL, and this was with Tarvaris Jackson throwing the football.
Now the Panthers do not have much money, so there is a chance that Seattle will simply match the tender. But Seattle does not have much cap room either, and they just lost Golden Tate. So would they match the offer? Probably not. Percy Harvin is healthy now, and Jermaine Kearse is a better athlete (taller, faster). So how much will they pay to retain a 3rd WR, especially when they would get a 2nd round pick back in return?
So why would the Panthers give up a 2nd round pick for a guy that Seattle is willing to get a 2nd rounder in return for? Simple: for the same reason why the Panthers tendered Byron Bell at the second round level when he wouldn't even be a starter for most NFL teams. Just compare the Panthers' depth chart at WR (just two guys the Panthers picked up from the practice squad) to the Seahawks. The Panthers are better off with a proven veteran for their 2nd round pick than a rookie who might turn into the next Dwayne Jarrett or Keary Colbert or Armanti Edwards. (Or Brandon LaFell. Yes, Baldwin is better than LaFell.)
It is a low risk move. Heads and Seattle matches and keeps Baldwin. Tails and the Panthers get a veteran WR for a pick that they were probably going to use on a rookie with more upside but less experience and far more risk anyway. Baldwin plus, say, James Jones or Jerricho Cotchery gives the Panthers their two starting WRs going into next season, both veterans who put up similar or better numbers to Steve Smith and LaFell. That frees the Panthers up to use the draft to concentrate on OL and the secondary, i.e. to get the most NFL ready OT in the 1st round and the best CB on the board in the 3rd.
Now Gettleman can't do this right away. There is not enough cap room to craft a contract offer that would prevent Seattle from matching. So if the Panthers offer something like 3 years for $11 million, Seattle will say "thank you very much" because that is less than they would have had to pay to retain Baldwin down the line. First he has to extend Greg Hardy, restructure or release some other guys to create enough room to offer Baldwin the going rate for young 2nd tier WRs: 5 years for $20-$25 million. No way Seattle would match that for a guy who could not beat out Sidney Rice last year (Rice got hurt, leading to more playing time for Baldwin) and may not even beat out Jermaine Kearse next season.