Even if Bitcoin is illegal in a country, the people of the country can invest or trade in Bitcoin. But there is an advantage if Bitcoin is legal in the country, Bitcoin can be used without any legal hassles.
There are pros and cons of being legal or illegal. Legal means citizens can use Bitcoin with law protection and if they get scammed, they can file reports to police, banks for help to get their money back. Illegal means citizens can use Bitcoin but law does not protect them with trading or investing activities. If they get scammed, they lose money, no one will protect them and support them to get money back.
The country I live in is Bangladesh, Bitcoin is not legalized in our country. No initiative has yet been taken that Bitcoin will be legal in the future. However, many people in our country invest in Bitcoin and many people do trading. Using cryptocurrency is a legal offense in our country. Cryptocurrency users are punished if the administration finds out, so we have to be very careful. We have an advantage that we don't have to pay any tax on our bitcoins.
Tax is another thing. No regulation, not legal yet, means you will be less watching in tax. Better protections with laws also mean stricter regulation on personal tax.