So I can use all addresses from the 'Addresses' tab?
Electrum automatically generate them all for privacy reasons?
Yes,you can use them because they are part of your wallet and they will always be in wallet.As I say before you can always use same address,but if you want more privacy then it is better that for every payment you use new address.Also when you want to send to someone for example 0.1
BTC and you have in total 0.5,Electrum will send 0.1+fee to destination address and 0.4 will be send to change address(for free).This is also option which protect your privacy,but it can be deactivated if you want.
You should take some time and read how Electrum work :
http://docs.electrum.org/en/latest/faq.html