GTFS can (and should) be the main means of connecting different software tools to ensure that information like stop locations, service headways, and paths of travel remain constant from tool to tool, and vendor to vendor. Since the GTFS spec is an open-source, standardized format, these programs can consistently import and use GTFS data. As such, managing data with a specification like GTFS lowers the opportunities for inaccuracies, and reduces the need for manually managing various types of transit data.
GTFS data can be used imported into a variety of transit technology tools, including scheduling, run-cutting, dispatching and vehicle location programs, and even with some on- and off-vehicle signage and annunciator hardware. Some of these software programs and vendors include:
Swiftly: Realtime provider.
Remix: Transit system planning tool.
Syncromatics: CAD/AVL provider.
Hastus: Scheduling/runcutting software.
Trapeze: Scheduling/runcutting software.
Connexionz: Scheduling/runcutting software.
TransLoc: Realtime provider and visualization tool.
[DISCLAIMER]: The US Department of Transportation does not endorse any of these software programs or their vendors.
Source: Swiftly Inc.
Source: GMV Syncromatics
Source: Remix
GTFS should be front and center for any transit technology vendor. If the vendor does not produce GTFS, allow import of GTFS, or believes that GTFS is not important, that vendor is likely selling proprietary or non-standard data. GTFS is designed to be able to be used by anyone - not any specific application or tool. While there are other means of translating transit information, such as through Excel workbooks or GIS files, only GTFS is exclusively built to describe transit services and be interoperable across data sources (e.g. your agency's GTFS can work in tandem with another agency's GTFS).