I feel dumb... But what's the purpose of Musketeers exactly ?

New player here.

I still struggle to see when i should be using Musketeers instead of Skirmishers and anti-cav units such as Dragoons and Halberdiers/Pikes. It seems everytime i go for them, they get absolutely shredded by artillery, skirmishers but also cav. I thought Skirm/Dragoon combo was more efficient.

How should i use Musketeers ?

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Thanks.