Here's why:

* GTP as an Energy Carrier: Like ATP (adenosine triphosphate), GTP is a high-energy molecule that can be hydrolyzed to release energy.

* GTP in Signaling Pathways: GTP plays a key role in many cellular signaling pathways. It acts as a molecular switch by binding to and activating proteins called G proteins.

* G Protein-Coupled Receptors (GPCRs): Many important receptors in the body, known as GPCRs, rely on GTP for their signaling functions. When a ligand binds to a GPCR, it triggers a series of events that lead to the activation of a G protein by replacing bound GDP with GTP.

* Signal Transduction: The GTP-bound G protein can then go on to activate other downstream effectors, like enzymes or ion channels, ultimately triggering a cellular response to the initial signal.

In short, GTP acts as an energy source and a signaling molecule in many important biochemical pathways.