In my eyes a landing page is important and should be started early on. Landing pages will change over time anyway.
You can always have some kind of sign-up for a newsletter or a waiting list. It's important to track the landing page (e.g. mixpanel or something like this). You want to know how many people come to your page, what they look at and where they click.
Your call to action must be clearly stated. The idea is that if nobody visits your landing page, nobody will get to your MVP either. If people visit your landing page but never click the call to action, it doesn't matter if whatever is behind the CTA is working or not.
A good landing page can also be a gateway to talk to people, collect contact data, ...