characters generally struggle with combat, mostly because of deckbuilding restrictions and class-wide scores of 2 or less. Any of them can mount respectable defense via toolbox cards like Knife but overall a will not be able to kill any sort of major enemy, ranging from Ravenous Ghoul and Hunting Nightgaunt to Umôrdhoth itself.
Typically you leave these enemies to someone else to kill or try to avoid them, but this kind of defenselessness isn't always viable even with a friendly face like Mark Harrigan in the party, there is an upper limit to how much damage a character can do in a round and sometimes you'll find yourself engaged with a big threat at a time where the other guy has something more important to do or is just outright busy with a danger of his own (such as having this carcosa dude and that other carcosa dude spawning simultaneously).
This is where "I've got a plan!" comes in. Hitting routine attacks is an issue for decks, even Shrivelling is only so accurate without something like Higher Education to boost it, but mounting one big attack by chucking all of those unused little icon cards that typically gather in your hand is a very reasonable bet. Boom, one hit and you've killed the 4-hp threat you just drew or contributed 4 points of damage to the boss.
"I've got a plan!" has it's weaknesses, your sustained combat ability is still not great, you need to draw it and you need the clues, but its a high-yield attack in a class thats otherwise bereft of good combat options. This is pretty much an autotake in every deck whose Investigator isn't the upcoming Joe Diamond.