there can be a few reasons, but it all depends on each player, of course. and most are good at making excuses.
1) he/she may want to erase the previous records by what he/she has done on the last char.
...possibly he/she was a criminal and might have enemies, dekinned players, so on. renewing reputation.
...possibly he/she has left scents on the previous char and has been marked by others.
...possibly he/she is embarrased of his/her previous char for some reason.
...possibly he/she was low lvl enough not to care about the main stats, but rather have a new char name.
...possibly he/she has died recently, and some mixture of the above.
2) he/she may just want to try and see if its the right village he/she is looking for.
...possibly he/she may have gotten a few invitations and want to join the best one later with his/her main.
...possibly he/she is just a bit too paranoid of getting main killed at the new village. (but it happens sometimes)
3) he/she may be a spy! hiding his/her true identity!!! checking out if your village got any good loot
4) he/she may be a thief! just going to grab all the valuables in your village and leave soon!
5) he/she may be the one you had kicked before!
what good can alts do?
well you can sometimes get to know the player without worrying about his/her relationship with other players.
you dont have to worry about rangers (or raiders) they may bring in by the scents they left recently.
when your village is established, you can feed enough curios and food to let them advance faster in the beginning and till they get good enough, they shouldnt mind doing chores other experienced players do not actually wish to do lol
(if they still mind doing chores while they can't do anything good, they are no worth, kick them.)
...hmm that is about it that i can think of right now.