Streak plates allow us to scratch a mineral on the plate revealing its streak. This can come handy if 2 minerals look alike. For example if you cant tell the difference between gold and pyrite you could use their streak golds streak is gold while pyrites is a goldish green.
Streak plates allow us to scrape off a chunk of a mineral to observe the colour of it's unique streak. a quite obvious example is gold's streak, which is coloured, well, gold. hope this helps!
