For the leather footwear that I use, I polish them with Bundeswehr boot polish once a week and brush molten lard into the once a month.
The old tale goes that the lard or other animal fats such as tallow help soften and condition the leather as well as waterproof it somewhat, which prevents the leather becoming brittle and cracking.
However I've seen it be argued that animal fats such as lard with weaken and rot the leather and this is the reason fats are removed from leather in the tanning process.
So what is the truth? Is the old-as-time practice of greasing up your boots beneficial for them or am I ruining them? Or am I being lied to so that I have to but new boots sooner?