This depends on five things: 

Are you concerned about traveling in Europe and would like someone to help guide you around?  Then a package tour that takes you to key sites might be perfect.

Do you enjoy having someone describe items of interest in a group setting?  If so, tours can be a fun way to experience a site or exhibit.  

If you are short on time, tours are a great way to avoid standing in line for an attraction because many of them can bypass the entry lines.

Do you have mobility issues, or a family with small children? If so, check how much standing and walking are involved.  Some tours are very strenuous, even involving some hiking.  Make sure you are physically able to keep up with the group.

Finally, if there are no more tickets available for the general population then tours may be the only way to get in.