If you have Flash Player installed, it downloads advertisements (from sites which have that type of ads). If you have an internet connection where you pay for megabytes, it may cost you more. If you have a slow internet connection, it may make it slower.
There may be some things you want to view with Flash Player, and if you don't have it installed, you can't view them.
You can use Flash Player only sometimes. To do this, put the Flash extension in the /tce/optional directory. To start it, use the Appbrowser to install local. Restart Firefox (or your internet browser) for it to work.
If you use Firefox, another option is to install Download Helper. This will download Flash videos to the computer instead of playing them in the browser. You can then play them in other programs, such as Mplayer.