The use of IVR-based systems is primarily to extract information from end customers without using software tools. The goal is to reduce the number of customer care representatives required to perform such large-scale operations as Service Provisioning of handsets.
Most of you might know that CDMA handsets do not use a Subscriber Identity Module (SIM) card to store service-specific information of the customers. The service plan purchased by each customer is necessary to identify the valid list of operations a customer is allowed to perform with his phone connection. In CDMA phones, such information has to be "programmed" directly onto the phone. They call this task "provisioning".
Imagine that a new operator (or exisitng operators) decided to roll out a new phone connection scheme that attracts a large number of people. Each person who purchases the connection would like his phone (existing or new) to work with the new plan (and fairly quickly - it's an unforgiving and fast world, you know :p). If the service provider has to arrange for customer care centers that people could visit to get their phones programmed and "activated", it would require some massive infrastructre and logistics.
To counter these problems, present day service providers use what is called Over The Air Functionality (OTAF). Using OTAF, programming bits are "sent" over-the-air to cellphones so they can program themselves. The OTAF box however needs details about the connection that a customer has purchased in order to decide what "bits" to send over-the-air. This information has to be input from the customer for which an IVR-based menu system is used. The Customer Service Center (which is actually a piece of software) sits at the service provider end and relays various voice-prompts to the cusotmer, expecting a DTMF response each time. A set of valid DTMF responses will take the (Finite State Machine based) menu-system to a the correct database record which has all the details about the purchased connection plan. The OTAF box is then used to send the derived programmed bits over to the cellphone.