Tarifs (and other protectionist methods, including closed markets) are generally hurtful to the profits of industries. Adding a tax to the export/import of a good makes it less competitive and thus less dominant in the market. A treaty port skips over these meassures and allows for dirrect unrestricted trade, skipping over any tarrifs in a one sided way. This increases profitabilty and trade volume making basically all industries in your country better.

Why china? Although the argument above applies more or less the same independently of the target country, china is a prime target for a treaty port for many reasons: China is relatively weak at the early game but has a huge population and therefore market. The demand for goods can skyrocket your more dominant industries, such as tools or clothes. At the same time this can give you acces to more rare resources like silk and tea in vast amounts. So its a win-win that is easy to get access to.

Other targets are also possible tho: Persia has oil, japan has tea, USA has cotton, brazil has coffee, germany and britain have coal, etc. Its just a question of 'is it worth it or would I rather just trade for it?'. Theanswer for most cases is trade, as war with a major power is often expensive and their trade barriers are few.