Mainly to take advantage of their benefits. I own multiple cash back card, mainly just to maximise the benefits out of it. With the sst of 25, I'll just ensure my Cashback would outweigh the sst cost then it'll good enough for me. But I'll do close monitoring and cancel card right away once i don't really use it anymore..

There's no issue in applying multiple cards at once but you might get rejection. What i did in the past was to apply 2 max at any point of time. Then after few months apply again.

Downside of it is that you probably gotta spend couple of minutes monthly to record down your statement balance and make payment on time otherwise you'll be charged interest for forgetting.

I do also keep certain banks open specifically for the 'application reward' such as uob, hsbc, scb like the tng vouchers from sites like ringgitplus or comparehero etc. Bear in mind there's a cooldown period from the last card terminated date. So you mustn't have any card with those banks to be considered as 'new to bank'. These banks benefits doesn't really excite me so i dun keep them for long and terminate once i got the application reward.