A corporate Seal is a protection against fraudulent activity. It has legal value (by default defends) in case anybody creates false docs or makes false loans in your company name. It's highly recommended if your company has frequent transactions or provides receipts and invoices. It's also needed if you go for a loan, mortgage, employment contract, service contract or any sub-contract.
Share certificates are legal and official proof of ownership of a corporation. Bank may ask to show it to open a bank account. Most people choose 5 share certificates to save costs; some choose 10 or 20. It includes blank certificates in case you want to add spouse, child or friend as shareholder in future.