On the plus side:
+ Inform users about products (which is a small convenience).
On the negative side:
- They provide unsollicited information, which distracts and irritates users.
- Users can also search products by themselves (no hand-holding needed).
- They use screen real estate.
- They use bandwidth.
- They use energy.
- They make websites slower.
- They introduce security problems.
- They track users, collect their data.
- They put users in an information bubble.
- Market failure: not the "best" product wins, but the one with the biggest advertisement budget behind it.
- They make people feel inconfident (see photoshopped fashion ads)
So, any value offered by ads is easily canceled by the negative points listed above.