The Type-moon wiki describes the creation/passing on (The way it's worded a Crest is functionally re-created/re-built every time it's passed on) of a Crest as a series of grafts carried out over time and probably starting early in life based upon Rin having the Tohsaka crest despite her father dying by the time she was 6.
So what happens if/when Shirou passes those circuits onto his child? (A Crest is normally only given to one child)
I'd say they were created to enable or do that, but that's not what they actually do. Remember that the crest is actually composed of Magic Circuits, which may or may not actually have spells 'loaded' into them. One of the problems that seems to face modern Mages is the sacristy of Magic Circuits, you only get what your born with and what, if any, you inherit via a Crest, and apparently those you develop naturally isn't normally that many, at least by the standards of things you may want to do.
And one thing I read suggests that Crest Magic Circuits 'loaded' with a spell are useless for anything else
I hate to bring this argument back up, but I just came across a source, so I felt it was worth posting it.
Source: Fate/side material (2004-1-30), p.073
An inheritance passed down within magi families.
The mysteries that a magus has spent his entire life establishing (stabilizing) are turned into a crest and left to his descendants.
Itâ€™s no exaggeration to say that the crest is engraved with the history of the entire bloodline, and that the magus who inherits it is burdened not only with his familyâ€™s regrets, but is duty bound to pass it on to a successor as well.
In a way, you could call it a curse passed down through the generations.
Note that it specifically mentions mysteries-- as in the spells researched by the family. Circuits are entirely secondary. I'm also going to state flat out since you mentioned it in another post-- don't use the TM Wiki as your main source. It has a lot of inaccuracies scattered about.