Assume I have checked an email using
is_email and the result is true, should I sanitize with
sanitize_email? if yes, then why?
Regarding the edited question, here's another old Q&A, which might actually be a better reference, Should I sanitize an email address before passing it to the is_email() function?, especially @kaiser's answer.
So to quote @Howdy_McGee's answer,
I would even go so far as to say that if this function returns true, you wouldn't need to sanitize it before sending it into the database.
But he also notes in the comments about using the sanitize function,
...better safe than sorry and the sanitization overhead would be entirely unnoticeable.
So based on these, I'd say that it is not strictly necessery to use
is_email() returns truthy value (the email). You could use it maybe out of habbit or consistency of sanitizing everything before saving stuff to the database (if, that is what you're going to do with the email).