Here's why:

* Sieve cells are specialized plant cells responsible for transporting sugars and other nutrients throughout the plant. They have a unique structure with sieve plates that allow for the flow of materials.

* Companion cells are located alongside sieve cells and have a nucleus and other organelles. They are metabolically active and provide support for the sieve cells, which lack a nucleus and most other organelles.

* Companion cells help regulate the movement of materials through the sieve cells by controlling the concentration gradients and providing energy for transport.

In summary: While sieve cells are responsible for the movement of materials, they rely on companion cells for crucial support and control.