Well the first main reason is that TOAD Data Point is an ODBC connecting client, that allows you to connect to virtually any database for building queries involving data from multiple sources (something I wish TOAD for SQL Server would allow me to do as well as connect to SQL Server!). However, you will not get the full features of SQL Server on TDP as you would using TOAD for SQL Server. Otherwise what’s the point of having two different products?
If you’re a SQL Server DBA, TOAD for SQL Server is paramount I think. However, if you’re a data analyst that wants to report data from various sources, including csv, xml, Excel, Access, etc then TDP is for you. You may need both TOAD for SQL Server and TDP if you’re also managing SQL Server databases, which is what I do, as I need to link client datasets from Excel spreadsheets with SQL Server, without the need to create tables in SQL Server and import data in from Excel (pain!).