If you were a GM on another team, and Glen Davis was unrestricted, why would you even do a S&T instead of sign him outright? And if you had to give up a little bit just for the comfort of knowing you got him, what level draft pick would you be willing to give?
Since I don't know what the next CBA will be like, I will answer this under the assumptions of the current CBA.
One reason to do a S&T is if you are over the cap and plan on using the MLE on someone else. Of course, if you did that, you would have to send out actual salary and not just a draft pick. Well, unless you had a trade exception you could use.
Another reason is if you want to give Davis the 10.5% annual salary increase that a team can give its own free agents rather than the 8% than a team can give free agents signed from other teams. That's probably worth at most a second-round pick, but Davis is unlikely to command a high enough salary where that matters.
There might be other reasons, but those are two off the top of my head.