Bitcoin is engineered for absolute scarcity and maximum security. Dogecoin is engineered for speed and low cost. Both are decentralized, both use proof-of-work, and both are held in the same kinds of wallets. The difference is purpose.
Think of it this way: Bitcoin is the asset you protect for the long term. Dogecoin is the asset you use without hesitation. Many people keep both in the same portfolio because they solve different problems.