I've recently seen that some companies use raspberry pi clusters as supercomputers. For example, Los Alamos National Laboratory has a supercomputer with 750 Raspberry Pi 3. And I was thinking if you can increase the number of cores so easily with a raspberry pi cluster why don't more people use them? The problem is that I don't know what applications allow multithreading. My browser for example does, so I could get a better browsing experience by using a couple raspberry pis. But what other common tasks could be performed faster on a raspberry cluster?
"what applications allow multithreading. My browser for example does, so I could get a better browsing experience by using a couple raspberry pis"*
No. A cluster is not equivalent to one big computer for any purpose. For starters, they can only share information over a network connection, which is orders of magnitude slower than the CPU communicates with RAM. So, a cluster of pis would be faster than a single Pi for performing some analysis of a large number of web pages at once, because each pi could analyze a page at a time and pool the results. But it would almost certainly cause your web browsing experience to become unbearable (unresponse, stuttering, etc)., not better, which is why no one in their right mind would try to implement that.
The relationship between "multi-threading" and "clustering" is not a simple one. Generally, interactive graphical applications use multi-threading so that the interface remains response while another thread is (eg.) loading stuff from the network. It can be used to improve performance speed wise, but there are limitations WRT when this is true and when it isn't.
If your question is, what would a cluster of pis be faster for than a single pi, there will be lots of answers like that, but the answer is not anything and everything.
"why don't more people use them?"
They are mostly (perhaps only) used for educational purposes. It is a cheap way for people to learn about clustering hands on. But this is because they are cheap, period, for a single system that can run a normal OS with standard network and peripheral interfaces. It is not because they are economical pound for pound, by which I mean $1000 worth of Pis clustered will not be better than a $1000 x86-64 system -- it is very much the other way around, which is why no one sells pi clusters for cutting edge gaming, etc. There may be some tasks for which this is not true but they would be very unusual especially in the context of conventional end user purposes.
The pi has a slower cpu, slower network, slower memory, and slower disk devices than a conventional computer. The network device on the pi3 can't even keep up with a regular desktop's network, and is slower than 1G. It is designed to be cheap and robust, not fast. There is nothing a pi cluster can do faster. In some circles, a pi cluster is considered a joke -- although a pretty cool one. Several pi clusters have been built for demonstration purposes.
The pi has more readily available I/O pins than a regular computer, so at a stretch, a pi cluster could do a lot more gpio. This might actually be useful of the pi's were distributed over a wide area; they could act like cheap (and slow) distributed I/O extenders, but this is not a conventional cluster. by any stretch of the imagination.
Los Alamos National Laboratory's Pi cluster's purpose is not to be fast. It is to test cluster algorithms. Sometimes you want a slower computer when testing code, because if it is too fast, things happen too quickly to debug. But also, 750 pi's is a lot smaller and cheaper and uses a lot less power than 750 conventional servers. Speed isn't everything.