You may be interested in this question on ux.stackexchange.com - has been asked last year, and has interesting answers (including one from @joelspolsky, who was on Excel team when this feature was designed): http://ux.stackexchange.com/questions/10135/why-are-there-th...

Perfect, thanks!

Glad it helped :)

Usability - It's a hint to those who don't already know that a workbook can have multiple sheets