1 It might be a sign of interest
According to body language, touching might be a sign of physical attraction. While the act of touching is not done out of sexual interest, it does still signify that the person wants to get closer to the one he is touching.
2 Showing sincere (non-sexual) care
A person can touch another person if has sincere, non-sexual, emotions towards that person. A person who feels comfortable talking to someone or who believes that he is speaking to a good person might touch him while speaking.
3 They know it can help
A study has shown that a person who gets touched before asked for a favor is more likely to help than a person who doesn't. People who know this fact might touch others intentionally.
4 To encourage a person
Many people touch others while talking to encourage someone and to show that their intentions are good. Inside the gym, for example, a person can touch the shoulders of another while telling him 'great job'.
5 They unconsciously think it's helpful
Even people who don't know exactly how a touch helps in facilitating communication still use touches because they unconsciously believe that they can make the communication with the other person smoother.
6 To make a person calm down
Many people touch others when they want them to calm down. This is usually seen during verbal fights where a bystander or a friend can touch a person while asking him to calm down.
7 To grab attention
Many people touch others when approaching them, especially when others aren't looking, to grab their attention. (See Why are some people are attention seekers?)
8 To create emotional closeness
Some people touch others to create emotional closeness while talking to them. This usually shows that the person who made the touch is sincere about his talk.
9 They feel comfortable talking to that person
According to body language, a person only touches another when he feels comfortable talking to him. In body language the smaller the distance a person keeps between him and others, the more likely it means that this person is comfortable around those people.
10 In need of emotional closeness
Many people touch others repetitively while speaking because they have a deep need for emotional closeness. By touching others, those people satisfy that non-sexual need for intimacy. (See 20 Signs That You Are Emotionally Sensitive)