Great work features that increase supply limit - actually a debuff?

[Astro-Boy]

There's a handful of great works features that increase supply limit (retinue and a bunch of great harbor features). I'm wondering though... in what way is this ever really beneficial? Really it just seems like it allows the enemy to use a larger army to siege my province.

Unless supply has an effect on siege defense that I'm unaware of, this feels pretty silly.

upgrade_cothon = { # Cothon
    local_modifier = {
        siege_defence = 0.10
        supply_limit = 0.5
    }
}

Quick example is the Cothon feature, which has the intention of improving siege defense and increases supply limit (can use the harbor to bring in supplies, I guess. makes sense). but in gameplay terms this is improving conditions for the besieging army, not the besieged.

Am I dumb or what?

 

[SigurdStormhand]

A larger Supply Limit allows you to mass a bigger defensive army in a province before you start to suffer attrition.

 

[Ambion Iskariot]

+50% Supply Limit usualy only Counts for your own Holdings. It works this way with any other supply Bonus, too.