Not having to wait 4-8 hours to access them. Also, pulling from S3 costs you nothing (if you're in EC2 or other AWS services). Glacier access costs can get expensive depending on the quantity of data you need to access in a short period of time:

"Glacier is designed with the expectation that retrievals are infrequent and unusual, and data will be stored for extended periods of time. You can retrieve up to 5% of your average monthly storage (pro-rated daily) for free each month. If you choose to retrieve more than this amount of data in a month, you are charged a retrieval fee starting at $0.01 per gigabyte. Learn more. In addition, there is a pro-rated charge of $0.03 per gigabyte for items deleted prior to 90 days."

http://aws.amazon.com/glacier/pricing/

To store 1TB for a year in S3 is now $368.64

To store 1TB in Glacier for a year is $122.88

We're quibbling over ~$250/year?