Hi Hiam,
You only need to install that version of the .Net Framework, if some software is not working without it and specifically indicates you need to install that version.
If that is needed to be installed, open the old Control Panel, then go to Programs and Features, then click 'Turn Windows Features on or off' and tick the box for .Net Framework 3.5 and install that, it will also contain the version 2 and 1.1 of the .Net Framework.