The play-acting helplessness is just part of the weakness and submission fetish. For a lot of AGPs there is a heavy humiliation component, especially those who are into sissification etc. I believe it is absolutely sexual, especially the way they talk about this, i.e "a group of big strong muscular men were eager to assist a helpless womanly woman!" I think the pretending to be extremely physically weak is just another manifestation of behavioural autogynephilia.
And as a side note I don't believe a man taking estrogen will actually lose much of any kind of strength, look at MTF athletes competing against women as proof. It's just part of the fantasy.