You can have as many user-defined variables as you would like in your front matter that can be referenced with .Params in your templating. You can read more about this in the documentation too.
.Title just happens to be one of the required fields in Hugo.
If you want to add something different, you have a lot of freedom to do so in your templating layer.
For example…
title = "My Article"
seotitle = "My SEO Title"
Then in your template
<title>{{ with .Params.seotitle }}{{.}}{{else}}{{.Title}}{{end}} </title>