renew command will take a look at all active certificates and renew those who are close to expiring - which is currently defined as 30 days before the expiration date. If your certificates aren't due for renewal yet, the client won't renew them.
The reason why a daily cronjob is recommended is in order to avoid issues caused by service downtime on Let's Encrypt's end, or any issues your server might have. If you, for example, run the cronjob just once every month or every two months, and the service just happens to be down during those times, you'll end up with an expired certificate eventually. By doing it daily instead, Let's Encrypt would have to be down for 30 consecutive days for that to happen, which is rather unlikely.