I have seen some sites with different Sub domains for different services.what is the use of it?

If I recall, you could in the past you could all the subdomains to rank for same query. But otherwise it's usually just to "contain" a theme for a domain/subdomain or for convenience

many benfits....organization.....and most notably you could have subsite.maindomain.com instead of maindoming.com/directory/subsite, or something similar...

most hosting companies use CPanel as their control panel that allows for easy subdomain creation

Because sub domains are part of the main domain name they don't need to be registered as they belong to the same owner as the main domain name.

Subdomains are effectively the same as domains but with another dot, for example if your domain was plasticsurgery.com, a subdomain could be my.plasticsurgery.com or your.plasticsurgery.com. They can lengthen your domain name significantly, and make it more difficult to remember, but the good news is with most of the lower-cost hosting services you can have as many as you want!

Sub-domains make your URLs shorter and nicer. It allows you to categorize content of your website.  Sub-domains also help in search engine rankings because they are treated as separate website address by the search engines.