High Relevance on Mobile Devices
Although the description is not very relevant in regards to keyword optimizing, its importance should not be underestimated. This is the especially the case due to the rising use of mobile devices, in which the description is displayed infront of the bulletpoints.
No Limitation - Summary of All Product Characteristics - Check Reviews and Question & Answers
The description should be filled out generously (at least around 200 characters) and include each and every information which may be relevant to a customer's decision making.
This should also include information which may seem trivial, such as the product being able to withstand certain tempuratures, or the fact it can also be cleaned with material x.
For this purpose it is often a good idea to check the reviews and questions & answers of your product, as well as those of competing products, to gain a better understanding which kind of information customers are looking for before they buy a product.
Space for Emotions & Personal Connection
The description offes enough space to further elaborate the emotional benefits mentioned in the bullet points. For example, a gamer could be persuaded to buy a mouse pad by describing a creative szenario in which the gamer's performance and thus respect within the clan was raised significantly due to far better control of the mouse.
Depending on the product or customer type it can also be a good idea to try to connect to her/him on a personal level. For instance, if the customer feels the seller is just as interested about the the topic surrounding the product, or about "how hard it is to find the right solution on the market", that may well be the final puzzle to complete the customer's buying motivation.
With the ability to offer long and informational texts in the description comes the need for a clean and thought-through structure. Text without structure is less likely to be read.
Google SEO Purposes
If a keyword is placed within the description, the product will be indexed for that keyword and can therefore be found in Amazon's search results. Keywords within the title, backend searchterms and bulletpoints will be valued higher by Amazon's ranking algorithm, so that it offers no additional value to add them to the description from an Amazon SEO point of view.
For Google SEO purposes it can help though to repeat the most important keywords in the description.