Spread is essentially Microsoft Excel, packaged as a control. It has practically all of Excel's functionality built in, including features such as charting, grouping, formula support, and more. Features unique to Spread include formula calculations using over 300 built-in Excel functions, Excel-like conditional formatting, and full featured import/export of Excel XLS and XLSX documents, including formulas, charts, sparklines, borders, cell formatting, and more.
The FlexGrid and True DBGrid controls, on the other hand, are traditional data grid controls. They offer great performance and customization, both in terms of appearance and behavior.
If you would like to have Excel embedded in your applications, then Spread is the logical choice. If you need traditional data grid functionality, then you should consider FlexGrid or True DBGrid.