Trade leads to economic growth.. It allows countries to specialize aka become really good and efficient at developing a certain niche of goods. Then exchange them with other countries.
If no one traded each country would have to produce EVERYTHING even stuff they weren't really suited for or good at producing. It would cost more to produce said goods and on top of that you couldn't export what you were really good or well suited for producing both of which are economic drains.
The fear is if TPP is passed among the Asian countries without the US, then those countries will have an economic advantage because they'll further be able to specialize compared to the US.
People on the center left to far left oppose TPP because they fear it will lead to low skilled/manufacturing job loss to overseas markets. Far right opposes it because it gives Obama increased negotiation power (usually congress has more oversight when it comes to the drafting of treaties). Generally speaking people who lean left (business democrats), moderates, people who lean right (fiscally conservative, socially liberal to moderate business conservatives) and center right fiscally conservative mainstream Republicans people support TPP.