When I bought my 2002 +8 it came with two sets of keys, both immobiliser plip type.
I checked both and all ok.
I replaced batteries on both with ones from a high turnover shop ( assumed the stock would not be old). One at a time just in case. My reasoning was I knew the age of the batteries. Changing was straight forward replacement, but I think there was a limited timespan to get the new battery in after removing the old.(?). Googled for instructions and precautions.
I use the spare occasionally to make sure it works and

has the spare set when we go away for weekends etc.
A bit belts and braces maybe, but peace of mind stuff for trips away.
I forget what the procedure was for a new key regarding programming it. With my Fords there was a procedure involving inserting the key into the ignition and going through a on/off/bleeps process. Had to do it once on a Focus and straight forward.