The body adapts to specific training stimuli over months. Same training year after year produces plateaus. Varying focus across years — strength block, hypertrophy block, endurance block, mobility focus — keeps the body adapting.
How to vary
12-16 week blocks of specific focus. Different but complementary. Example yearly cycle: strength block (Jan-Apr), hypertrophy (May-Aug), endurance (Sep-Dec). Or run-focused half-year, lift-focused half-year.
Variety produces both better results and more interesting training. Most plateau from monotony as much as overtraining.