what's the significance of certain moving averages?

i understand what moving averages are, but i don't understand why certain parameters are used....

for example, the 4HR MACD strategy has a bunch of MAs...like 21, 89, 200, etc.

why those numbers? why is it significant when prices start to find support at 200EMA? can't we just come up with a random number like 142EMA and when price finds support there to say it's also significant?

thanks.

Yes, which is why indicators are not keys to unlock forex. Sure some may show some movement or overbought/oversold levels, but there will always be some MA or some StDev setting that shows "well price bounced because the 143 SMA crossed the 12 EMA when the 154 WMA is below the 62 EMA"

The power of MAs, and specific periods, is in their popularity. Seriously.

Indicators almost have a self-fulfulling-prophecy built into them. The more people see them, the more they are likely to manifest the predicted result. I believe this to be true with Fibonacci levels. Many traders see the price hit 50%, so they think they price goes up, they buy and therefore the price, indeed, DOES go up.

Same goes for MAs. It is believed that many high-level analysts use the 200SMA as a barometer of price. Therefore, a lot of people use the 200 SMA.

I personally like the crossovers. There's little doubt that crossovers will indicate a trend. In fact, I've created a profitable EA based on this and posted it here: http://www.forexfactory.com/showthread.php?t=145135

Well I can't really comment about a specific strategy I am not very familiar with but the 200 will give a longer term view of the trend and as you go to bigger MAs you tend to use round numbers and probably when the system was being developed 200 gave a good number of touches.

The 21 and 89 are Fibonacci numbers and that may be why they are preferred rather than 20 and 100

You can plot any number and any type of MA and you will see price behave around it. Dont worry to much about values of indicators they are not of ut most importance.
However if u want to be in and out of the market fairly quick, obvoisly u dont want to use the larger MA's. Develop your tools around YOU, not the other way around.