MSS vs MTU

Hi!

In a Cisco environment, why would we use ip mtu instead of ip tcp adjust-mss under a tunnel interface?

Let's say we have IPsec over GRE tunnels. It looks to me that ip tcp adjust-mss is a better option because it tells hosts in a communication to lower their packets so they don't get fragmented.

If we use ip mtu 1400 for example, how would the hosts know to send smaller packets ? If they send 1450 bytes packets for example, they would get fragmented because of the 1400 mtu on the interface.

Thanks