Although the leg pieces are cut from pressure-treated lumber, the cuts expose inner untreated wood to the ground, allowing moisture and pests to enter. Benches which are placed in dirt or grass are especially vulnerable. Treating the end cuts with a wood preservative/sealer helps prevent this.
That said, decay has not caused problems so far, so this step is optional.
There are many products available for sealing end cuts. The most standard is copper naphthenate (“CuN”), which comes as either an oil-based or water-based solution. Oil-based solutions such as Copper Green and Tenino also act as a sealant since the oil repels moisture. Water-based solutions such as Woodlife CopperCoat are easier to clean. Copper naphthenate has a strong, lasting smell, described by a volunteer to be “really nasty”, so we no longer use it for volunteer build days.
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Borate-based solutions (Timbor, Bora-Care, PenaShield) should be avoided; without additional sealing they will leach out when it rains.
For more information, see chapter 15 of the USDA Forest Products Laboratory’s “Wood Handbook”.