Advertisers are in control of whether they show their ads for brand queries. Google has certainly been pushing for it, but there are many legit reasons for advertisers to bid on brand terms, even when they rank highly themselves.
A couple off the top of my head:
- Helps push competitor's down that might bid on or rank organically for your brand term
- Gives you control over sitelinks (you have much less over organic sitelinks)
- Helps build account-level Quality Score which ultimately helps control CPCs account-wide
- Let's you control your message
- Let's you control your audience (such as by using RLSA to retarget to specific audiences, exclude others, etc.)
- Gain a MUCH deeper level of understanding on how people are searching for your brand in context with other touchpoints
- Gain some very good data on brand query volume and composition that could lead to other insights
The list goes on. There are definitely some cases where it isn't necessary or warranted though. However a savvy marketer will weigh the pros and cons and not make a snap judgement that "brand terms are bad" since they are often worth the minimal expense.
Source: Do this for a living and helped lead the paid search group at a top search agency