Is the water cold? Maybe fish a nymph deep & slow. Is the stream low, clear, & on the warmer side of a trout's ideal metabolic range? Perhaps a dry fly on a fine diameter tippet will be the way to go? These are just the basics, but the water temperature can tell you even more.
Let's say you arrive to fish a spot that you know always has trout there, with the idea that it's a sure bet to catch at least one or two. But something has happened, you're not picking up any action. You can't even see a trout. What happened? Well, careful & constant observation of the water temperature can tell you what might be going on.
In the summer, trout will migrate to cooler water, usually upstream. The ideal temps for trout are usually in the upper 50's to the mid-60's - a little less for Brook trout & Rainbows & Browns can take it a bit higher. When the water temperature in a stream reaches 75 degrees & stays that way for 5 hours or more for a few days in a row the trout will begin to migrate. This is why your favorite spot might be empty - even if the water temp is acceptable when you arrive to fish. No, the spot is not "all fished out" as you might hear some anglers say. Rather, the trout have all left for cooler waters elsewhere. Now go find them with your thermometer.
There are exceptions to every rule, of course. The oxygen levels of the water can allow trout to withstand higher temperatures,so take everything I just said in the paragraph above with a grain of salt. Highly oxygenated water will have the trout feeding actively, even when the water temps are much higher. But as a general guideline, water temp is usually a great indicator for trout presence & activity in any given stretch of water.
Taking the water temp is easy & fast. Just make sure that you hold your stream thermometer in the flow of water - make sure it isn't touching the bottom or any rocks, etc. I've rigged my thermometer up with a long length of old fly line & a wrist band, so I can't drop it or lose it no matter what happens. Hold the thermometer in the flow for at least 30 seconds. Remove it quickly from the water & read it immediately.
If you're fishing without taking the water temp, you're fishing blind. If you get into the habit of reading the stream's temperature before your fishing begins, you're setting yourself up for more success along the stream & will be a much more educated angler.