Apollo.io / sengrid noobie question

I am more of a DNS & eMail guy.. Not Mass eMail Pro on all platform.
MY QUESTION :
Some consultant told my customer to use sengrid when using apollo for his eMail campaign(to send out eMails)
What do you think ?
Sendgrid is a MUST / NO brainer ? Or not needed…
I guess apollo default way of sending eMail is good too ?
Note : as I said I know a bit what apollo does but don't even know if when using it, we are forced to use some external eMail service ( MTA), from what I know, no….