Partly that's true, and it's a mechanism for kicking the can down the road for another two years because we've failed to sort everything out yet.

There are legal reasons too though. During the transition period (assuming it happens), we will have legally left the EU. Which means we can actually sign agreements with it as an outside party. And we can negotiate and sign those much-vaunted trade deals with non-EU countries, to come into effect at the end of the transition period. We can't sign any of those things while we're a member.