What does it mean to be chosen as a Contact?

It's totally your choice. No need to do it out of politeness. See the following links for details regarding Contacts:

http://help.yahoo.com/l/us/yahoo/answers/network/n...

http://help.yahoo.com/l/us/yahoo/answers/network/c...

http://help.yahoo.com/l/us/yahoo/answers/network/f...

http://help.yahoo.com/l/us/yahoo/answers/network/c...

http://help.yahoo.com/l/us/yahoo/answers/network/v...

If any user finds other users questions or answers interesting,helpful,informative,knowledgeable and useful enough then the first user can add the other user as a contact,it also means that he likes the answers of his/her contact and is a fan of his/her questions or answers.

In fact the benefit is just as a 'bookmarking' so that you can easily find those contact's answers in the future in your needs,as the contact's avatar will be added to your Y!A page's profile's 'Contacts' list, but there no points added to any parties score for adding contacts.

Typically people who agree with you are added to your contacts. You can delete them if you choose.

It is just a way for people to find you easily again. ~

They have found that they may want to check your future answers, because they liked what they saw of your previous one(s).

like Heinz said you had ask a very interested question and thats why they wanted to add you as their contact,you don't have to accept it but thats up to you.