Dead cells that hold the plant up
Xylem does two things at once, and it does them with the same cells.
Transport
Transporting the water with mineral salts absorbed by the roots of the plant throughout the plant body.
Support
Providing support to the plant.
Why one tissue can do both
A lignified wall is stiff, and a tube with stiff walls is both a pipe and a pillar. The same thickening that makes the vessel strong enough not to collapse under the pull of the water inside it also makes it strong enough to hold weight. The plant gets its plumbing and its scaffolding in one structure.
Wood is xylem
The trunk of a tree is very largely xylem — layer on layer of it, added a ring at a time by the cambium. That is why wood is strong, why it is made of long fibres running the length of the trunk, and why a plank splits more easily along the grain than across it.
Support is a function of xylem, not of phloem
Both are complex tissues, both run in the vascular bundles, and it is easy to write the same list for each. Only xylem supports the plant. Phloem has one function, and the next skill gives it.
Water up, and the plant held up
Xylem transports water and dissolved mineral salts from the roots throughout the plant, and provides support. Three of its four cell types are dead, lignified, and doing both jobs at once.