They are used early in the sourcing and procurement process – just one of the many processes in the organisation of a conference or convention.
A request for proposal is made for a specific product or service and many require suppliers to prepare their responses in a certain way to provide a structure for sourcing or purchasing decisions. This makes it easier to compare offers, but it is not always useful because it limits suppliers' options for making creative suggestions. You should provide scope for this in RfPs.
Also bear in mind that you will probably have to send RfPs for a wide range of services, such as transport to and from the venue, accommodation, equipment, staff, printing etc., so consider carefully how many suppliers to contact in each category. Firstly, you will have to read and compare them and secondly, it does not always make sense to contact a large number of potential partners because the range may already be limited by certain requirements. Alternatively, you can save a considerable amount of time by returning to suppliers you have already worked with successfully and whose prices are fair.
There are various other ways for potential suppliers to get onto your RfP list, such as recommendations, previous occasions when you were impressed with their work or enquiries in other departments.