It depends on where your website is hosted, and how you’re making these changes. Some web hosts (e.g. GoDaddy Managed WordPress) will make a copy of the uploaded CSS file to improve performance and serve it from somewhere other than where your WordPress site lives. So, in order for you to see your changes, this file has to be re-copied into the cache, which you can do by clearing the host’s cache.
If you’re just working on designing your site in the first place, you will be able to get work done a bit faster if you work in an environment that doesn’t have this kind of caching turned on, such as a local development environment or a “staging” copy of the site that doesn’t have caching.
