theres a few reasons!

first, it makes it nice n easy to replace the quite thin and bendy hacksaw blade. you got more room around it to do so!

another cool thing is it also allows you to remove the blade, insert the blade through a hole or something, then reattach the saw.

for example, take a bit of box steel, maybe like this:

(photo from here)

lets say you are in a pinch and need some angle iron, but don’t want to just take an angle grinder to it because maybe you need it to still be precise (enough). this is our end goal:

(photo from here)

first, you’d take out the hacksaw blade, which is just a thin little metal thing. after that you’d put it through the tube, and then reattach it to the saw. if the saw is now stuck inside the tube, you’re on the right track. now you may saw away!

here’s another good albeit nondescript example:

(photo from here)

above all though, we’d say the best thing about the design is that there isn’t usually even an entire inch of blade height inside whatever you’re cutting. this means there is a lot less friction and that you don’t need to keep the blade perfectly straight for every stroke beyond the first few.

plus, when the blade is well in there, you can twist the rest of the saw a little bit, and the blade will simply act as a spring and continue to cut straight! it’s pretty neat.