I saw some people that share their mail like name at gmail dot com instead of name@gmail.com
Why does they do it?
This is not something that only Gmail users do, it was a very common practice on "forums" and billboards as an attempt to prevent that the emails were harvested automatically.
IMHO it doesn't make sense nowadays. To learn about this topic you might want to read the following Q/A from other sites from the Stack Exchange network:
- Does e-mail address obfuscation actually work? from Super User (nowadays closed)
- Is using 'dot' and 'at' in email addresses in public text still useful? from Information Security
- Does e-mail obfuscation really make automatic harvesting harder? from Stack Overflow
@symbol is provided in a web page.