What is a minifier?
Minifiers remove unnecessary white spaces, tabs, line breaks and everything else that seems meaningless. Some minifiers also rename the variables so it can save you even more space (and also giving you some extra protection against malicious users that want to steal your code).
Why should I care about using a minifier?
Minifiers are very important in software because of the performance benefits it brings with them. Nowadays, almost all websites and mobile apps interact with a server on the Internet, whether sending or receiving messages. If the size of these files can be reduced, then websites and mobile apps can load faster, reducing bandwidth and money.
Also, minified files are meant to be read by computers, not by people. So if the computer doesn't matter if a file is minified or not, we will prefer to minify them (because of the benefits described above).
If your are starting a new website, consider that Search Engines (such as Google or Bing) rank your website better if load time is minor.
Is a minifier the same that a compressor?
No. Minifiers and compressors are different things, but are used for the same purpose: reduce file size. While minifiers reduce file size by stripping unnecessary data, compressors use algorithms to reduce file size (such as gzip and brotli).
However, nowadays, many people use minifier and compressor as synonyms, but consider that they are no the same thing!