A diode is essentially a one-way valve for electricity. What you probably saw it used for is polarization protection. Basically, so that if you accidentally connected your battery the wrong way, no current would flow, and your electronics would be protected. (Not all electronic components need polorization protection, it just depends on the parts.)
An inherent characteristic of a diode is that it will have a voltage drop across it when connected in series to a load (your motor/esc). Typical diodes have a 0.7V drop. This 0.7 voltage drop steals power away from your motor/esc. Instead of your motor/esc receiving 7.4V from your 2s lipo, it would only see 6.7V.
This is why you'd want to use a Shottky diode. A Shottky diode only has a 0.3V drop.
Hope this helps.