Why does this happen at 6-box as opposed to 5-box?

3 and 3.

I was going to say that it made sense visually but of course, it's not always 3 reds and 3 blues. It just seems the right time to do it.

But they do the squashdown and then immediately take out another box often leaving a hole in one side which then sits there at 5-box whilst the offer is given. Surely it makes more sense to do it at 5-box when it'd be much neater looking?

I suppose it's intended to heighten the tension a bit during the selection of the last box of that round, and highlights the significance that last box could have on the 5-box offer - something some people might underestimate!