Cost-Effective
The prices for top-tier quarterbacks are out of this world. FSU saw that when they were in the Cam Ward sweepstakes last year. FSU paid a hefty price for DJ Uiagalelei, and it didn't work out, but FSU also had a terrible offensive line.
Castellanos wanted to come to FSU, and they may have gotten a hometown discount, which freed up funds to address other positions.
Frankly, it didn't matter who FSU brought in at quarterback if they didn't get the offensive line to at least competent levels.
If you're potentially losing a bit at quarterback but able to address multiple gaping holes on both sides of the ball? That's probably an advantageous trade-off.
It also likely gives FSU a greater margin of error at quarterback if the offensive line is better, skill positions are better, and the defense is better than last year.
It may be possible for FSU to take a second portal QB and help build towards the future once Castellanos leaves too.
Let's be honest. FSU will not likely compete for an ACC Championship or 12-team College Playoff. However, taking Castellanos may allow FSU to ascend quicker in the big scheme because it allows you to address multiple issues while getting an experienced guy that fits your system.