Because you basically have no control where the money will be taken from to create the payment.
You want to explicitely tell your wallet that it should use all those 6 SPR blocks as input.
This is not guaranteed when you use sendfrom, sendtoaddress etc...
Except ofcourse if you guarantee that the SPR address you are sending to is not in the same wallet where all the 6 SPR blocks reside,
or else the next sendfrom might take from this 3000 (or whatever) SPR address instead of using the 6 SPR blocks.