Why apple tell me this machine Maximum memory is 8 GB
There are a number of components in the MacBook Pro 13" Unibody Late 2011 that can be cost effectively upgraded.
Memory: The MacBook Pro 13" Unibody Late 2011 comes with 4 GB RAM standard, and accepts a maximum of 16 GB. If your MacBook Pro is still running with only the stock RAM, upgrading will provide a dramatic performance boost. You can upgrade to any combination of two 4 GB or 8 GB modules for 8, 12, or 16 GB of total RAM.
Max Memory Size (dependent on memory type) 16 GB
Memory Types DDR3-1066/1333
How should I choose？
Don't you love Intel's double speak ;-}
OK lets look at this from the CPU size first: The i5 & i7 chip in this series can address 16GB of RAM. But, that doesn't address what combination of memory modules you could use. Hence Intel's (dependent on memory type) comment.
It's also important to note only the newer MacBook Pro systems can support 16GB of RAM. Here's a list C-Net MBP systems that support 16GB of RAM.
Now the down side:
Installing 16GB of RAM will be very expensive and most users would never even need to access that much. You would need to be working on some very big data files or DB's to leverage that much memory. It also will suck down your battery life and make your lap a little more toasty (hot).
So is it a good thing to be able to support than much RAM - without a question yes. Just don't run off doing it yet. Over time the cost will come down and maybe you might need it for some future OS or application. Besides, over time we have seen how chips get smaller and use less power (and heat) so waiting is a good thing from that point as well.
Buy what you really need for most 8GB is more than enough.