The cut end of asparagus spears is known as the woody end for good reason. It’s so fibrous that it’s virtually inedible. It’s not essential that you trim it off, but it makes sense to get rid of it so that all of the asparagus that makes its way onto your plate is delicious. Otherwise you’ll end up eating all the way up to the stringy and fibrous part, then leaving the rest on your plate.
When you bend asparagus spears they naturally snap at the point where the tender part meets the woody part. If you’re preparing lots of asparagus, you might want to snap one or two to get an idea of where this is, and then use it as a guide to trim the rest with a knife. While at first glance it feels quite wasteful, remember, even if you left it untrimmed, you wouldn’t be able to eat that part, so it would still end up in the trash.