I might sound stupid..

[Sugarplummy84]

...but why must purees be frozen in an ice tray? Please explain :)

Because those tiny cubes are about all they eat at first so that's all you'd want to defrost.

Its the right sort of portion size, I think people pop 1 or 2 portions into a bowl to defrost it, thats all.

But it should have a lid on it.

I got some individual tiny pots with lids from poundland and Asda.

It's a convenient portion size.
Freeze them quickly in the ice cube tray and then once solid, pop them out into a big tub with a lid and lable it. Then make more of a different sort. Then you can mix and match to make a different taste or texture etc.

you dont have to use ice trays, you can freeze them in anything you want. Ice cube trays give a good portion size though.