There are three things you must have in your sidebar of your blog: search bar; archives, and category.
Let talk about search bar first, why I say it must be added in your blog. OK, I ask you, if you want to search something from other blogger’s blog, suddenly you found that their blog don’t have search bar, then you cannot search the information that you want. What you need to do is dig out the archives to read their posts. How many people will going to do that? I tell you, I will leave this blog and not interested in their blog anymore.
Why we need category and archives in our blog? If a blog without category, this is means that their blog posts are posted with unorganized and not in good manner. Some people include category but they even don’t know how to label their posts in suitable category. Anyway, it is good to have category in your blog rather than no category.
All the posts will be stored in archives, people can refer to your archives to read your old posts. If you don’t put archives in your blog, then we need to keep pressing the “back to page” or “older post” link to read your old posts. Please, blog without archives will also make me quit immediately.
So, remember, a “standard blog” must have Search bar, archives and category, otherwise you don’t expect people will read your blog or even stay longer in your blog.