An example below of why it helps delivery if we know that you are ready.
Regulating traffic within a particular apron, for example the old Charlies...124 calls for start and is transferred to ground, then 143 calls straight after and transferred over as well. Now these two might be still sat in the ground pending bay 10 mins later having not yet called for push. So what do you do on delivery when 126 calls you for start? Its unfair to delay this one who might be fully ready, but you can bet that as soon as you transfer it to ground, the other two will call for push as well. If ground R/T is already busy, he/she can do without these calls (don't forget there maybe inbounds to get into the apron also).
The ideal situation for delivery is that you call up when you're just about ready to push. That way the start ups can be regulated properly with regard to Ground workload, too much traffic at the holding point, runway alternation, slot times etc, as is the job of the delivery controller.
If you're not yet ready and just want your clearance, say so, I'm sure 99% of controllers would pass it to you.