Scenario, you are building a system and you are on a budget. You come to the decision of what RAM to buy.
Your board has four slots for RAM and the myth which is propagated is that you have to populate all those slots with identical RAM otherwise you will be doomed for all eternity.
So instead of investing money sensibly, such as in a good PSU (Power Supply Unit - VITALLY IMPORTANT) or a better graphics card (optional) you waste your money and buy four sticks of RAM, just so that you can be sure your system runs in the future although it could run perfectly well on two sticks of RAM now.
So here's what you do, you get two sticks of identical RAM and put them into separate RAM banks. Sometime along the way you want to increase the size of your RAM you then look for the best price for RAM which has the same timings as your original RAM and then take the original RAM you bought out of the second bank and put it into the rank of your first RAM and put the other two into the second bank and rank.
SORTED! As long as you have not bought inferior RAM, the system will run NO DIFFERENTLY AND WITH NO PERFORMANCE LOSS, compared to you having bought all identical RAM at the same time at the higher price.
To run optimally a system should have identical RAM in the same Bank. This does not however apply to RAM running in separate Banks.
What you have to remember is though that the RAM in the separate Banks will only run as fast as the slower RAM installed, so although you don't need to get identical RAM from the same manufacturer, you do have to make sure that the RAM will run at the same timings and speed.