In its purest form, an SSD is flash storage and has no moving parts whatsoever. As a result, they're smaller and take up less space in a PC case, in some instances even mounting directly to the motherboard. SSD storage is much faster than its HDD equivalent.
HDD storage is made up of magnetic tape and has mechanical parts inside. They're larger than SSDs and much slower to read and write.
If you're building a PC for any purpose, you're going to want speed. If you only have HDD storage in your machine, then speed isn't something coming your way. Windows will take longer to boot up, applications will take longer to load, and files will take longer to open and save.
The beauty of an SSD is that this waiting time is dramatically reduced, even on cheaper drives, when compared to HDD. Anything you load on it will perform actions much quicker.
One of the best uses for an SSD in a PC is as a boot drive. This means installing a small-ish capacity drive on which your Windows 10 operating system will live and boot up from every day. By doing this, your PC will boot up and be ready to go in a flash. You can also put your most frequently used software on there for a similar effect.
The lifespan of an SSD isn't as long as that of an HDD, but you can certainly use an SSD as your only drive in a PC. Many laptops only have SSD storage, for example. You're not looking at a short-term life, but compared to HDD storage, it is reduced. SSDs don't handle being written to as well over time, but a boot drive wouldn't suffer this.