Mosses and their relatives are seedless nonvascular plants, they are called bryophytes. They lack a vascular system (plumbing system) to move water and nutrients throughout the plant. For this reason, they grow close to the ground to absorb water and nutrients.
Seedless plants also rely on free-standing water for reproduction. A sperm produced by a male must swim through rainwater or dew to reach an egg. The diploid the undergoes meiosis to produce haploid spores. These spores may also need moisture to disperse.
The Crash Course video, The Sex Lives of Nonvascular Plants; Alternation of Generations, goes into the life cycle of these early plants.