And that's the other side of the coin right? In most trades you have to give to get and unlike some here think, GMs aren't trying to screw over other GMs in trades, rather they are just looking to improve their franchise. So in most cases, teams do value those they trade away. Example: I believed TB valued Howden and Hajek when they traded them but they valued McD more for their situation at the time.
Now we don't know the specifics of how the Buch/Blais trade came about. Who reached out to who and for who? We don't know why the Blues made the trade or the Rangers for that matter but we can guess.
Same thing with the Blues outlook on Blais. Looking at his usage over the last three seasons, I believe the Blues view him as a good tough to play against winger that played well enough with ROR and Perron but one they could easily backfill so they packaged him with a 2nd to a team specifically looking for that type a player for what they see as a good 1st/2nd liner that could also be in their PK rotation.
Interesting note: In the 2019 playoffs and during the two following reg seasons Blais played limited minutes but did indeed play the majority with ROR and Perron. In the 2020 and 2021 playoffs he ended up playing most of his time on a more traditional lower line with Sanford and Thomas (then Clifford). He also has never been in their PK rotation.