I think there were two reasons. The first was to protect Hoffman. This may be the reason Kerry was killed, too - at least, she had figured out there was a second apprentice just before she died, and I think she might have been getting too close to Hoffman. The trail was there, as Strahm found in V.
The second reason is that it was a kind of test for Hoffman. Unlike Amanda, Hoffman passed and so survived. Strahm was more or less doomed from the moment he disobeyed instructions and left the room at the beginning.
I've just come back from seeing it, so I haven't processed it all yet and these are my first impressions. I found it a bit slow at first, but I enjoyed the middle section. Like other people, I was disappointed by the lack of twist at the end. I think if people hate the whole thing then they aren't really into the franchise anyway.
There was a lot we'd already figured out. The pig-masks disguise appearance. Initially we think it's always Jigsaw behind the mask, then we find that sometimes it's Amanda, and from IV we know it's often Hoffman. It seems Hoffman was the first apprentice and it would have been interesting to see more of Hoffman and Amanda together. (I'm assuming Amanda at least knew about him, since he had helped eg with Kerry's angel trap.) It was fun to revisit some of the setups from earlier films, but we didn't really learn a lot.
It seems to me now that Jill is definitely evil, because she claimed Strahm was following her and I don't think he was. So she was helping to put suspicion in chief Erickson's mind. I am expecting the VI film to be some kind of show-down between Jill Tuck and Hoffman.
I hope VI is the last. I was a one-off. II showed they could do it again. III, IV, V and VI seem to be planned out together as a quadlogy. I hope they have a definite and satisfying ending in sight.
Still don't know what is in the note Hoffman wrote for Amanda - I suspect now we won't find out.