The MLAG feature allows you to combine ports on two switches to form a single logical connection to another network device.
Why couldn't I do the same with etherchannel? Make 2-summits stack and give a link from both to a required host. At host's side configure teaming or NIC bonding.
I don't understand the diffrence, please, explain.
It's about downtime. When you upgrade a stack, the whole stack must be rebooted. When you upgrade a pair of MLAG-Switches you can upgrade and reboot one by one. With the right design you can achieve a always up core.
Besides what André already said, When you stack, the control of the stack is made only by the master control plane. The other switches are serving only the ports. On a MLAG configuration, both control planes are active and processing, so load is shared, and processing capability is added.