Yeah, if you want the money in your GIA you can always remove it from the ISA and pay it into the GIA and you’ve not lost anything by doing that apart from time.
But the benefit of having it paid into the ISA is that you get additional money in the account without affecting your annual limit, so for most people it’s beneficial.
The only downside for me if that it then looks like the ISA is performing better than it really is because the balance is increasing every month by ~£10 and that money is taken from your bank by DD, so the “up since you began investing figure” is slightly misleading (it excludes deposits and is supposed to represent earnings from investments).