Bilt’s Rent Day offer this much is a 50% or 100% Transfer Bonus to BA Avios.
We don’t know why some accounts get one or the other. It’s easy to pass on 50% since a 30-40% transfer bonus is common with other programs.
If you have 100% and you are an experienced Avios user – meaning you don’t need to know how to use them and you already frequently do, then you’ll probably want to move some over. How much depends on how often you use Avios vs other airline currencies, your Bilt balance, and your other point balances.