Perhaps the better question is something along the lines of "why add LqdOxygen"? If the only answer is realism then that's insufficient. Time for a long blurb.
In my current gameplay paradigm (which has evolved over time, but is stabilizing), a new fuel type only brings value if has particular gameplay concepts that are different than other fuels'. A lot of my mods did not build on this, which is why it's a bit of a mess in NFP (fuels that shouldn't exist by this paradigm). The effective gameplay (in a non RO context) ways of differentiating fuels:
Density: affects ship design significantly in terms of ship size
ISRU factors: affects mission design if the production chain for the resource differs or if it is limited in presence
Boiloff: affects mission deign and adds ship design-based mitigation strategies
Transport factors: transferability and similar affect how the resource is actually used (eg solid fuel)
Cost: a poor differentiator because a) KSP cost balance sucks and b) sandbox doesn't care
Tank mass ratio: a poor differentiator as it is effectively a scalar on mass and Isp.
Engine selection: a poor criteria but should be mentioned because it drives reasons to use the fuels. For example, Oxidizer is a bit more valid of a fuel because LiquidFuel can be used in some cases without Oxidizer.
Typically IMO a new fuel type should not be added unless it provides a new challenge when used. Ideally it should hit at least 2 of these factors. My current fuel lineup looks like this, and you can see why I am only partly happy with it based on the above:
LH2: Fairly good differentiation with 2 strong and 2 weak factors (boiloff, density, engine selection, mass ratio)
Argon: moderate differentiation, probably shouldn't exist - 1 strong, 2 weak (ISRU, cost, engine selection)
Lithium: Poorly differentiated, should never have added it (ISRU, engine selection)
Uranium: Decent (ISRU, Transport, cost, mass ratio, engine selection)
Now adding LqdOxygen and looking at it in these terms:
Similar density to oxidizer so not great
Presumably same ISRU properties as LH2/Oxidizer/LF. if different, would need more models/work
Boiloff as LH2 - slightly different as presumably slower
Same transport factors as any liquid fuel
Cost not considerably different
Mass ratio not significantly different to Oxidizer
Engine selection limited to LH2 engines - already a gating factor to LH2 engines
The only real reason that works is engine selection - and to effectively use this, you would need to use this resource in other engines that are non-LH2, because said engines are already gated by LH2. It is my considered opinion that using LH2 in these engines is basically only a naming change and doesn't really affect gameplay. Hope that answers your question :D.
That being said, the boiloff code completely supports multiple types of fuels and is customizable to do whatever you like, as per one of the items in the FAQ specifically addressing LqdOxygen.