Topping it off

I would like to hear from others that have experience and knowledge about top coats.  I think this topic would help a lot of DIY's.  I have seen a few questions people have asked why things have happened to items that I felt a top coat might have prevented.  I admit this is not my field of expertise (not sure I'm an expert at anything, lol) but being I sell cabinets I do understand somethings.  I think knowing the different results from high gloss to satin etc. would be helpful to know as well as top coating painted items with or without glazes.  I have not worked with much of this but hope to get into it much more this summer.  Any opinions or tricks to getting a good top coat would be appreciated, or maybe you don't think one is necessary.  I would like to hear your thoughts.