From my understanding of the complex rules, this offers Cassel a little less risk, but is not a major factor in changing the ultimate outcome.
He gives up the opportunity to negotiate directly with any team, come Feb 27, by signing the tender. He is under contract, like any other player. Without signing the tender, Cassel is free to negotiate with any team to seek the highest bidder and best situation.
However, he still has some control, because he can refuse to negotiate a long-term extension with any team he doesn't like, should he, I don't know, sign Archie Manning as his advisor. So, while he loses some flexibility, he still has some ultimate control. But, worst case he now has a guaranteed $14.7mm. You can pee on his leg, but you can't take that guarantee away.
The Patriots now know that, unless they trade him, they owe him the full $14.7mm. However, their ability to trade him is unchanged.