Why frozen peas in shepherds or cottage pie

[Gfplux]

I see in almost every recipe of Shepherds Pie to add FROZEN peas on the top of the "mince" then the mashed potatoes.
Why frozen?
Having cooked shepherds pie (with beef mince as one of our family hates lamb) a few times, I see that if the "mince" is too wet there is a tendency for the (real) mashed potatoes to sink in to it.
So I have the beginning of a theory that the frozen peas create a support for the mash.
Anyone else have thoughts about this.

Why frozen peas rather than tinned?

Because tinned peas are vile. Do you seriously serve people tinned peas??

I didn’t even know tinned peas were a thing! They sound vile.