Aside from price, customers want to assure themselves they are indeed purchasing the items with the attributes they wanted. For apparel, this is usually size and color. For other types of products, it might be size, flavor, shape, memory or any other item that differentiates a sku. If the cart needs to be adjusted to lower the cart total price to fit the budget, or to eliminate near-identical products the customer was considering during the browsing process, it's all the more important to be able to identify what items were added to the cart -- through image thumbnails and other indications.
Bad cart summary pages don't provide enough detail:
"Hooded Sweatshirt" is kinda vague, and without a thumbnail image it's near impossible to figure out which sweatshirt is which save the item number (not helpful), size and price (not necessarily helpful).
OK cart summary pages show a thumbnail:
Good cart review pages indicate sku attributes like color and whether the product is in stock:
Excellent cart pages show the sku color, large clear thumbnail images, stock availability, tax and include a shipping calculator pre-checkout.
The above example is from Eddie Bauer, one of the best checkout pages I've seen.
The more you can reduce customer anxiety on the cart page, the less abandonment you will experience. The more user-friendly your cart page, the less anxiety it causes.