I am in the process of making a tote bag. The instructions in several places say to sew 1/2" seam then trim to 1/4". I fail to understand why that is necessary? I mean I realize in THIS particular instructions if I changed it now the pattern size would be off. But what I don't understand is why it is necessary to begin with? It seems like extra steps. Can anyone enlighten me?

It might be easier to sew a 1/2" and trim if the pieces are tricky to put together. I really can't think of another reason.

It's easier to sew a wider seam, half inch or 5/8" in heavier fabric. Often, the instructions will tell you to 'grade' the seam after sewing. Usually you cut one layer down to about a quarter inch and the other layer of fabric a bit wider. This reduces bulk and makes the seam lay flatter. Cutting the entire seam down is less usual, but again it's difficult to sew a narrow seam in heavier fabric or a bunch of layers. Here's a youtube video on grading seam allowances:
http://www.youtube.com/watch?v=_gseV1cF9o4

I agree with the previous post. Usually you end up grading the seams to reduce the bulk.

Thank you PaperPrincess. This make perfect sense to me. I am using Peltex and sewing two of those layers together, so I am sure it will be easier. Do you think I should grade these seams versus trim all to 1/4"? The pattern I am using has been disappointing on several points so I don't see any reason to follow her instructions on this point either. Lol!

It is much more difficult to align all the layers and sew an even 1/4". In the greater scheme it isn't very wasteful to sew neatly and then trim.

It sort of depends on which seam it is and how you are going to press it. If it's a side seam, and you are going to press it to one side or the other then I would definitely grade it. If it's the side seam and you are going to press it open or not press it at all then grading isn't going to help. I'd grade the bottom seam and press it to one side. Peltex is a bear to work with, but the bags made with it look so crisp & professional. I love how they stand up on their own! Please post the finished product, we'd love to see it.

Wouldn't the size of your item be off if you only sewed 1/4 inch to start? Maybe that particular part is supposed to be a bit smaller so the designer has you sew a 1/2 inch to pull it in then trim your seam to remove the excess bulk. Or not.

I would think it allows you to top stitch a 1/4" without any problem. Sometimes it depends on the type of fabric you are using. A lot of purses and bags are top stitched to accent them, or if using a contrasting thread, maybe silk, it would be beautiful. Of course, your pattern says to trim down, but if your fabric unravels easily, I would suggest overcasting it or serging it. Good luck with it.

Could be the fabric that the pattern calls for. Also could be in a stress spot that is tight (or going to be tight), so they want to be sure the seam is secure. I find that the first time I make something, I try to follow the directions the way they have been written. After I have finished, I can start making any changes I want. I have patterns that I have changed seam allowances up or down, for my personal sewing comfort. JMHO