Baltimore offered Lamar 5 years, 250 million with over half fully guaranteed at signing. It would have paid the most annual money of any QB and was the 2nd most guaranteed money at signing ever. Lamar wanted the whole thing guaranteed because that is what Watson got. Baltimore isn't going to fully guarantee 250 million dollars to a QB that has now been injured at the end of consecutive seasons and for multiple games in each such season (and I'm not sure anyone else would either). They will however franchise tag him for the next two and pay him over 100 million dollars to do so. If Lamar holds out, so be it, but that is around 50 million dollars he won't get, a lost season on top of that, and almost certainly won't make some team give him a massive fully guaranteed contract.
As for why Lamar should play, he needs to end the season not injured and on a potential high note. As has been mentioned in this thread and other places, Lamar hasn't exactly lit the world on fire in the playoffs thus far. If he can go against Cincinnati and have even a decent game, it would go a long way in showing why he is worth a fully guaranteed contract. If he doesn't play, then why would anyone pay him a fully guaranteed contract if he can't play in the biggest games of the year, has never performed well in those games, and can't finish seasons healthy.