Once you're on a decent HRT regimen your libido may drop.
Many report it crashing to the ground, some report maintaining some sort of libido.
The lowered libido is more a result of lowered testosterone than an increase in estrogen.
Sometimes the testosterone levels go lower than that of a natal woman.
(Even natal women are known to have libidos.

)
There are a few reasons to masturbate while pre-op.
Libido plays less of a role in some cases.
There is an old wives' tale that maintaining continuous experience of orgasms makes it easier to have them post-op.
Nobody knows if this is actually true or not.
Another reason is the atrophy and shrinking of the skin on the "raw material".
Although insufficient material for SRS is not commonly a problem, the threat of having to have a skin graft is cause for concern for many.
There is a tendency in younger people for the skin to shrink if not stretched in some way periodically.