Why are databases often used on eCommerce websites?

All business applications are SCREENS + BUSINESS LOGIC + DATABASES

Microsoft SQL Server 2008R2 received the most license purchases for 2011 - though other competitors are Oracle Database, DB2, Teradata, Informatica. MySQL is the most popular open source (free) database platform, though it is specifically geared for website back-ends. However Microsoft SQL Server Express is also free, and includes advanced services such as SQL Analysis Services, SQL Reporting Services.

From a business intelligence standpoint, Microsoft SQL Server BI Suite leads in the ability to execute, while SAP edges Microsoft in market saturation.

MySQL

As a side note, while Microsoft Access may not be as powerful as SQL Server, it is in fact the most installed database platform by far, though the contention that it is actually a full database platform is arguable, it can hold millions of rows, capping out (.accdb file) at 2 gigabytes.

See the related links.

Do not use Oracle Database, it is ridiculously overpriced, and none of what you buy comes bundled, so you if you want to tune your database, the tuning pack costs $5,000 per processor. And even for an average IT person, Oracle will take you four times as long to install, because it's poorly packaged.