Who has more chakra.

[lanakui8]

I said for healing and not for attacks. Remember when Sakura was trying to keep Naruto alive? She run out of her base chakra and she couldn't use her Byakugou chakra to heal him. She had to pump his heart to keep him alive.

why then didn't she just enter byakugou mode and heal him?

That's merely an assumption. Both Onoki and Gaara were out of chakra when Tsunade arrived to the battlefield. She healed them and restored their chakra reserves.

It's not an assumption, it's how the databook explains medical ninjutsu works.