Why is it necessary to ask a Rabbi?
In various places on this site, including the FAQ, we have the following disclaimer:
Like Wikipedia, this site makes no guarantee of validity, and does not offer professional (particularly rabbinic) advice. Treat information from this site like it came from a crowd of your friends.
This disclaimer is motivated by the concern that people might consult this site for personal advice about what Judaism says they should do instead of consulting their own Rabbi. Along similar lines, many people here include in answers about Jewish practice a recommendation that the reader consult their own Rabbi.
What is the reason for this concern?
Why is it so important to speak, in particular, to a Rabbi (and especially to "your local" one)?
What would be so wrong with simply using good-looking information from a site like this one that we need disclaimers all over the place and constant warnings?