Necessity of MoU for researcher exchange or collaboration
MOUs serve several purposes in my experience. First, they put down in writing what the two labs have agreed to do together. This formalizes the relationship a little and allows the participants to refer back to their commitments. Sometimes people get busy and forget to do some parts of the agreement. The MOU allows the other side to gently remind their partners that everyone committed to something and ask them when they will start on their part. People don't generally intend to shirk their responsibilities, but academics and researchers get busy, so informal and unpaid promises are first to slip. MOUs which formalize the relationship are often enough to make sure each side holds up its end.

Second, they allow others inside and outside the respective organizations to recognize that the relationship exists. Labs have directors, their directors have bosses (often department chairs, deans, or research vice presidents), and their bosses have bosses on up the chain. Having an MOU between labs establishes a formal relationship that can be recognized by these people. In fact, depending on the organization, the boss of the lab director may have to sign off on the MOU (not just the lab director themselves), which gives the agreement some level of officialness.  

This official recognition generally doesn't get used for much more than promotion of the relationship between the labs. Though, it may also serve as the first step in a deepening relationship between larger parts of the overall organizations (departments, colleges/schools, and even whole universities or government entities). These formal relationships can also be helpful in pursuing government funding in some countries to help support the collaboration. 

I've never used an MOU to formalize a simple summer student exchange. Those, in my experience, were always done with a verbal or email agreement. However, everything longer than a summer or more complicated than a student exchange that I've been involved in has had a written MOU. They lay out the expectations and give the signers cover with their higher-ups when the questions come about what two groups are doing together. 

It's not generally seen as a contract, but does up the seriousness of the relationship to have an MOU.