Sure, I can explain.
The majority of psychological experiementation is behavioral. We have learned a great deal regarding the connection to learned behavior vs. innate behavior, and how to address the issues of learned behavior via they processes through which it occurs through animal experimentation of this nature. There are certain things that are restricted ethically in the use of a human population. For instance, we have learned a great deal about addiction by the study of animal behavior, and it has led to more effective theory and treatment when it comes to the addiction issues of humans. However, we could not have conducted the same studies on humans because of ethical considerations. Without the research and the findings, humans would not have received the benefits.
When researchers utilize various species of animals, they take those variables that create differences into consideration and control for them. So a mouse not being able to burp would not be an issue, unless they were testing a drug in which burping was necessary for the drug to function as intended. In which case, they would use a species that could burp.
The fact is, with meds and surgical procedures, experimentation is necessary to insure relative safety for use in humans. That experimentation either has to occur with animals, or with humans. Personally, I would rather see a drug tested on an animal prior to it being approved for use in a human than have to be overly concerned that someone came up with something in a laboratory that hasn't been tested, and is being prescribed on a wide spread basis.