What do Roads and Train Tras do?

Can someone explain to me what does these do on the country and is it worth increasing its index to high levels or not?

Thank you.

Roads and trains help move goods around between your corps and your population in your country. I keep my transportation around 130-132 and it works fine for me.

It is part of the calculation for welfare index. Welfare index is determined by the lowest part of the formula. The formula is in documentation. You need transport at 140-150 to maximize its effect on index. Welfare index affects production of all corporations in your country so it is very important to maximize it.