Why is my washing powder not to be used for pre-wash?

Apologies but I can't find a more suitable stack exchange website to ask this question. I feel the technical answer is chemical in nature so I landed here.


For at least the past two years (and on-and-off longer than that) I've been putting a little bit of my washing powder into the prewash compartment, as well as the main compartment in my washing machine, and then doing a pre-wash before the main cycle.


A few days ago I noticed on the washing powder I had been using there is a red cross over "Wool/silk/prewash" - Implying that I should not prewash (at least not with this powder)


My clothes have seemed fine all this time (except there has been the odd occasion where I've been slightly paranoid that they are not being washed as well as they could be) but I wonder if I am damaging the clothes and/or the washing machine by doing this. 


