Ussd is used for unstructured data. What is unstructured data means? In telephony protocols all data formats are define in specs. E.g IMSI, which is the identity of subscriber, is well structured in a 3gpp specification.
USSD carries user entered data to network node, which can handles the ussd request. E.g Balance check. The user can enter the data for which no format is specified in 3GPP. Because of this it is call unstructured data.