Why do I need more than one wallet?

Hi, I'm a complete beginner in the bitcoin exchange field and I was wondering, why do I need more than one wallet? I heard having a cold wallet is the safest option, but why then do I see people recommending more wallets? Shouldn’t I just be careful and cautious while having bitcoin in the long term? If I invest I wouldn’t want to split the risk because I wouldn’t want to lose any of my bitcoins. Is it as risky as people say it is or is it that with proper knowledge I will be able to keep the bitcoins safe all in one wallet?