There are several reasons you should hit the button instead of writing it.
#1: It removes unnecessary forum space and keeps the threads clean.
#2: It adds credibility to the person you would like to thank in the way that other people can see how many times he or her have been thanked.
#3: People that don't have time to read every single post and looks for answers can easily see what post that's most valued and what holds the answer or good respond.
P.S: You need to have minimum 5 posts before beeing able to use the thank you button.