As there are few countries that have put BTC as legal tender?
Yes, El Salvador and Central African Republic (CAR) .
What does this mean for the system?
It means that in these countries it is used for transactions, payment of taxes and any other business that requires financial commitments in the country. It also mean that it attracts investors in to the country who are pro-bitcoin.
Does this make BTC more centralized?
No, it doesn't. Bitcoin will remain decentralized for as long as it is existence. You can read about its decentralization that cannot be altered in the bitcoin white paper. -https://bitcoin.org/en/bitcoin-paper
Is making BTC legal tender a benefit?
Absolutely. I already wrote early about it attractiving investors to the country which will improve the economy of the country and job creation. They will also be increase in adoption of bitcoin and then you become your own bank in every sense of the word. You are not going to be afraid that anything will happen to your money because you are in custody of it.
If say all the countries in the world did this? Could BTC be more controlled and centralized?
Bitcoin is not maintained by one entity but by several entities through nodes scattered all over. Bitcoin cannot be centralized. Read the Bitcoin White Paper
Or would it stay decentralized?
It stays decentralized.
Is making it Legal Tender just a way to offer acceptance. As a way for the government to seem friendly and let the people have better access to their own financial means?
Yes. It shows a government that is progressive and wants to free her people from the problems with that faces the traditional financial system one of which is currency devaluation