When to use a bridge note...

I've seen a number of entrepreneurs  using bridge notes as an alternative to seed or series A rounds – they’re using them when they do not have sight on a real institutional round. Bridge notes  should not be used as way to finance a company without putting a valuation on the company.
In my opinion, convertible bridge notes are best used when the next round of financing is visible and the company needs a bit of cash to hold itself over to the next round.  I do believe that's why they're called "bridge" notes.