Often, the assumption is that bidding higher on products means they appear more often. Vice versa, lower bids assumes that products won't appear as often.
However, bidding higher or lower also impacts the search term depth that triggers your ads in the first place.
To put it more simply:
Bidding Higher: Your ads are more likely to appear for broader terms that match or closely your product titles
Bidding Lower: Your ads are more likely to appear for specific terms that match or closely match your product titles
Real World Example
You sell t-shirts and one of your more popular products is a black t-shirt with a skull on it.
Right now, this product's title is structured as such:
Men's Black T-Shirt | White Skull Print | Size XL
When bidding higher for this product, its ad is more likely to appear for terms that are more broad in nature, such as just T-Shirt.
When bidding lower for this product, its ad is more likely to appear for terms that are more specific, such as:
Men's Black T-Shirt: Still somewhat broad but note the additional values for Gender and Color
Extra Large Black Skull T-Shirt : Further less broad, note that the Size and term "Skull" are now emphasized