Buying stocks in company merging/acquisitions Does an acquiring company have to buy all the stocks of a target company to acquire/merge with it?
If so, why? I thought stocks give voting rights and dividends, but not direct control of company management.
If not, why do they usually do it?
Stocks/shares are equity (a share is 'a part of' something) and are thus essential to ownership of a company. Owning shares means that you own part nof a company. The ownership, as a side effect, gives voting rights on the C-suite and dividends. 

Acquiring a company requires you to buy all of the shares of that company (so that you own it).