I wouldn't say that doing frequent backups will "keep your blog safe" but it is most important if disaster strikes. For example:
You ugrade to a new release version and find most of your beloved must-have plugins are now broken. With a backup you can roll back to the prevous version and keep the blog live while you look for alternative plugins.
Your blog gets hacked and polluted with SPAM comments. With a backup you can fix the problem and restore your comments back to before they were polluted.
In any case, frequent regular backups are very very important.