PO is same as Contract. Please note that PO is one type of contract. Most of the organization use different terminology to term the contract.
As you rightly said, PO/Contract is required for any purchase of Goods/Services.
The key criteria for classifications of contracts are;
1. Risk – Associated with impact of your organization status, brand and business.
2. Spend – High or Low
3. Frequency of purchase/annum – High r Low
PO is more used for low risk, spend and frequency of purchase.
Master Agreements/Master Contracts are used when the risk, spend and frequency of purchase is high.