Starting at 3rd level, you can use a bonus action to make a Perception check to spot a hidden creature or object or to make an Investigation check to uncover or decipher clues.
Suggests that a bonus action specifically will be useful - meaning that the Investigation check will be useful in a combat or other time-crunch scenario.
Uses for Investigation checks
Taking some examples from the DMG on how an Intelligence (Investigation) check might be used in combat:
Perhaps your party is being chased by a powerful adversary, and you need to escape a dead-end room. DMG 104:
Opening a Secret Door. Once a secret door is
detected, a successful Intelligence (Investigation) check
might be required to determine how to open it if the
opening mechanism isn't obvious.
Perhaps your party is engaging an enemy in a battlefield that the enemy had the chance to prepare. Detecting and disabling the traps for yourself or your allies is key. DMG 121:
If the adventurers detect a trap before
triggering it, they might be able to disarm it, either
permanently or long enough to move past it. You might
call for an Intelligence (Investigation) check for a
character to deduce what needs to be done, followed
by a Dexterity check using thieves' tools to perform the
There's plenty more ways, depending on how combat goes. You might need to quickly figure out how to use the MacGuffin to annihilate the baddy, or spot a clue as to their social ties that you can use for leverage.
The Swashbuckling Rogue Rakish Audacity feature:
You also gain an additional way to use your Sneak Attack; you don't need advantage on the attack roll to use your Sneak Attack against a creature if you are within 5 feet of it, no other creatures are within 5 feet of you, and you don't have disadvantage on the attack roll. All the other rules for Sneak Attack still apply to you.
Only really works for one-on-one fighting, you can have neither allies on your side, nor additional enemies. While it is certainly powerful, it's not quite an always-free Sneak Attack.
And the Panache feature:
[...] If you succeed on the check and the creature is hostile to you, it has disadvantage on attack rolls against targets other than you and can't make opportunity attacks against targets other than you. This effect lasts for 1 minute, until one of your companions attacks the target or affects it with a spell, or until you and the target are more than 60 feet apart. [...]
This feature occupies a different niche than the Inquisitive Insightful Fighting feature. Panache is more of a support feature, making the enemy less likely to hit your allies, while Insightful Fighting helps you deal damage directly. The value this brings to you depends on your team composition and tactics.
However, you're right in that the Inquisitive subclass is altogether weaker directly as compared to the Swashbuckling subclass. Initiative and Attacks of Opportunity are all things that take part in every combat, whereas having the combat depend on your character quickly identifying some tipping point is a lot more circumstantial. If you want your Inquisitive Rogue to use their class features in combat more, and in a meaningful way, you should talk to your DM about it. The examples above show that there are ways to make Investigation useful in combat, it then falls on your DM to give you these opportunities.