Most primers are sealers also. Primers generally stick to the substrate (wood) better than most finishes and most finishes stick to the primer better than the substrate. A sealer doesn't do much of what the name implies. What a sealer is designed to do is to raise the grain and seal the wood from raising the grain again. Anyway, that is what I was taught 40 years ago and I do realize that things do change over time. Today, most pre-finishes are primer-sealers combined.