Internship gives you indispensable perspective on research.
Increase your chance of getting a position in a lab: you can gain a noticeable amount of experience just by doing a 2-4 month internship. P.I.s know this, and take it a sign of both enthusiasm and competence. Furthermore, if you like the work at the lab and did good work, no matter the result, your P.I. can offer you to join the lab as a graduate researcher. This would be a good and easy transition into graduate research.
Adjust to Masters or PhD faster: at most times there is a noticeable gap between a person with internship experience and those who don’t. You can get around the lab and adjust to tasks a lot faster since you have the basics down. To be concrete, in a biological lab setting for example, you can learn: how to use a pipette properly, how to present your data in a scientific setting, understanding the social norm of a scientific lab. You will be better prepared to start Masters or PhD and save you time in the long run.
Gain perspective on which lab to work in and which project to take on: it is difficult to know what kind of project you can enjoy working on without a reference or if you will like scientific research at all for that matter. Internship will serve as this reference. Next time you choose a project to take on or choose a lab to do your graduate research, you will have much better idea of what is best for you.
Down-side of doing an internship: you have to put time and effort into doing an internship and though intern gets paid in most situations you may find yourself having to take on an internship without a salary. At most times there will be opportunities to apply and receive scholarship or bursary for a summer internship if the P.I. can’t or wish not to provide salary directly from their funding. Bottom line, internship will give you a head-start in research.
It is hard to gauge something you have never tried and internship will be a great experience in the long run no matter how it goes.