I think the public is well aware that many-to-most abductees report that an implant is inserted into them during an abduction. Many people are also aware of the work of Dr. Roger Leir has worked with abductees and who has assisted in the surgical removal of alleged implants. So what is the purpose of these implants?
Some speculate that the implants are being used to monitor a person's biochemistry, hormone level, etc. For example, an implant may be used to monitor when a female abductee is entering ovulation so that she can be abucted and have the egg harvested. Honestly, I don't know if this is a possibility or not. When it comes to alleged implants I can only speak from my experience and from what I was told by the aliens.
I know little about the implants, only that a biopsy or tissue sample is taken and somehow incorporated into the implant prior to it being inserted into the abductee.
I only have conscious memory of being implanted once, as a child, and while spending the night at my grandmother's house. I awoke in the middle of the night to what I thought were "sounds of a party". I heard shuffling sounds in the living room, as if a lot of people were walking around. When I sat up in bed the sounds quited down. I went back to sleep. I dont' know how long I was asleep but I awoke to the sound of the lamp in the living room being knocked over. I immediately sat up again and saw the light pattern from the lamp still moving or rolling on the floor. I'm not going to reveal the entire abduction experience but a white being ran into my room, and I do mean ran, and other beings joined it. One being had a silvery, metallic, thin rod that they inserted into my nostril. The rod reminded me of the rods on a rabbit ear antenna for some reason, except that it was far thinner. In a later abduction experience I asked about this and was told that it was done "to become one". An image of a small metallic ball with holes in it was projected into my mind. Small threads or filaments began to grow out of the ball and seemed to connect to cells or something. After this I knew the ball was "dead", that it had served it's purpose and would later be expelled from the body.
I suffered a lot of headaches and bloody noses, especially at night, as a child. The episodes were very severe. I somehow believe these are connected to the implant but I have no way to be sure.
So as to the purpose of alleged alien implants, again I can only speak for my experiences. I was told it was to "become one". There may be different types of implants for different things; however, I have no knowledge of such.