I read some people use the Google SMTP server to send their emails (when they use Google Apps for your domain). What would be the benefits for me to use MailCh

I read some people use the Google SMTP server to send their emails (when they use Google Apps for your domain). What would be the benefits for me to use MailChimp / AuthSmtp / SendGrid / PostMark? Where do I get the best chances to have my emails reach destination? Thanks