Soaking is economical — with a load already damp and the dirt softened, you can use less water, less detergent and less agitation in the wash cycle; it's gentler on fabrics, threads and colors; and you only need one final rinse instead of two.
Pre-treating also helps clothes last because high dryer heat makes stains set in permanently. I keep spot and stain removers next to my laundry hamper upstairs and pre-treat spots before they go in the hamper. Rinse the spot product out in the nearest sink if letting it sit until laundry day might bleach the fabric. You will see right away if you need to retreat it, or use a soft brush or a different product to completely dissolve the stain and save that favorite shirt!
Before soak-cycling in the machine on laundry day, sort well, so that you can treat each load in the appropriate way during the soak cycle. Treat spots first. If rhe whole load needs treatment, such as for whitening, or dissolving mildew, heavy dirt or grease, be wary that some whole-load treatments like chlorine bleach or Lysol poured straight into a tub of wash will stain or bleach clothes beyond repair. So fill the washer tub first, measure and mix the treatment in well, and then dump in the clothes.
Pre-soak treatments include:
• spot treatment for rust stains
• liquid, spray or gel treatments for blood, food or grass stains
• liquid or spray treatments specifically for oil or grease
• powdered oxygen bleach or liquid chlorine bleach for a full load of whites
• a capful of Lysol for mildrew or body odors (careful—undiluted, it will bleach colors)
• a tablespoon or so of powdered borax or TSP for especially dirty or greasy work clothes.
Many people have good results pre-treating stains with lemon juice, or adding white vinegar to the soak or wash cycle. I notice that the price of both white vinegar and chlorine bleach has gone up 150% to 250% since the pandemic. Lemons are always expensive. The price of OxyClean powdered bleach increased 15% - 20% at first, but has come back down somewhat.
Many machines have a pre-soak cycle that goes automatically right into the wash cycle with the same water. If yours is like that, remember to add your detergent when the cycle changes; or you will have to add it with the treatment at the beginning.
I prefer a separate soak cycle that then drains out the dirty soak water, and refills clean water for the wash-with-detergent cycle.
Nothing beats really hot water to get whites clean (other than drying outdoors in summer sunlight). To me, it's money well spent instead of dingy white clothing. My machine lets me choose "cool", "cold" or "tap cold". If it is really cold outside, "tap cold" will be the coldest and cheapest to run. If your dark load is medium colors with no dark or black colors that you really care about (like old dark gardening pants that are faded anyway) and it's really dirty or stinky, choose "cool" or even "warm" for the soak.
Dry powder detergent clogs up our pipes but ymmv.