While greeting some elders during any occasion or in a wedding or in temples, we join our palms together in a namaskaram to pray or to greet. This signifies respect and humbleness and hence we are taught from our childhood to do this.
Other than just symbolizing values, there is a lot of science involved in it. Our body has many pressure points that help activate a lot of organs. Likewise, your palms have a lot of nerve endings and hence joining both hands ensures joining the tips of all the fingers together; which are the pressure points of eyes, ears, and mind. Also, the moment you place your hands together, your dualities, your likes and dislikes, your cravings and aversions, all these things are leveled out and all the energies are functioning as one. Pressing them together is said to activate the pressure points which helps us remember that person for a long time.