A place for wash water, not a water source
Site prep sometimes calls for a spigot for finishing and washout, and that is easy to mix up with a washout pit. They sit at opposite ends of the same job. The spigot supplies clean water for finishing tools and rinsing equipment. The washout pit is where that water goes once it has picked up cement and fine concrete particles and needs somewhere contained to settle, instead of running off across the yard or into a storm drain.
Not where old concrete gets hauled
A washout pit handles wet material only: leftover concrete from the bottom of a chute, the rinse water from tools and hose, and whatever comes off a truck or pump at the end of a pour. It is not a place for broken up slab, old driveway, or any other demolition debris. That material gets cut out, loaded and hauled off as its own line of work, separate from a fresh pour, the same way tear-out is handled on any job that includes it.
What is actually in it and why it matters
Wash water off fresh concrete is alkaline and carries fine cement particles in suspension. Left to run across open ground, it can burn grass, cloud a ditch, or carry sediment straight into a storm drain the same way an unmanaged bare patch of a site does. Contained in a lined pit, the solids settle out and the water can evaporate or be dealt with properly instead of leaving the site. On a small job that might be a shallow, lined depression dug for the day. On a bigger site it can be a more substantial setup that stays for the length of the job.