British thread gauges measure threads on parts that follow the specific British thread gauge system. In the U.S., you're unlikely to see these threads unless you have imported equipment. However, that equipment is still pretty common despite being technically obsolete. You may be more likely to see U.S. customary or metric measurements in this country, but don't assume you'll never have a use for British gauges.
Equipment That Uses Them Still Exists
Through a combination of imports and good maintenance that helps British-threaded items last longer, you're likely to come across equipment that uses British threads and thread gauges at least once, if not more times. If you ever need to replace a fastener on the equipment, for example, you'll need the right thread gauge to ensure that you get a correct replacement part. These parts are likely to be around for a long time, so keep those thread gauges on hand indefinitely.
The Later Ones Did Use Metric Measurements
Some British thread items did start using metric measurements. That being said, there's no way to tell when you get a piece of equipment whether the threads use metric measurements or the older British thread sizes unless you purposely look or someone tells you. If you assume everything uses metric measurements, you'll end up without the right thread gauges. Not being able to measure threads properly means the equipment may not be able to continue functioning well due to mismatched or unavailable parts.
It's Easier to Replace Them Than to Try to Convert
You could go on a mission to convert all the British fasteners and other parts you see to customary or metric parts, but it's much easier to simply replace the British threads when the parts wear out. So, again, you'd need the thread gauges to see if the current parts could be used a little longer or if they truly needed to be replaced, and then you 'd need the thread gauges again to test the replacement parts. Even if you don't use them often, it's simply a good idea to have British thread gauges available to you so that you aren't stuck and unable to measure something.
You can get British thread gauges from many suppliers. There are a few different types, so be sure you look through the listings and get the ones you think you'll use most, given the type of equipment you work with.