Remembering previous videos, I'm pretty sure it's to lower Martha's health so she does her ult more often to speed up the party. Boosting Martha's health makes her less powerful.
There's no sense spending your gold on that, until: -All of your heroes are A* -All of your heroes' skills are maxed -Your titans' seals are maximum level
this is not compelty true, i get over 100M dmg against Osh 160 without buffs and with 920k meta team, theres much more factors about Osh fights, one thing whos really important is guild morale !
I'm surprised to hear that your guild hasn't beating 160 Osh yet. I think even Hero Wars Central mentioned his guild clears 160 and he's in a silver league. That said, I don't know how his guild manages that...
His guild didn't make top 50 last week that much I can confirm. My guild should have beat 160 several times already but there are certain players who can't follow instructions or wait until 160 is before burning their stonecold line. It's really frustrating.