Congratulations !
Of course you don't need an external flash as there is a pop-up flash on the D3300.If you do some research online you will realise there are limitations to using the pop-up flash mainly because it
only fires direct frontal and this causes hard shadows appearing on subject's face or body.An external flash would also do the same thing because it's sitting on the camera's hotshoe but why want to use it?
1. More power can cover wider and reach further (subject to flash's power output capability.)
2. flash head can tilt up/down and rotate 180 or 360 degrees.Therefore can bounce light off ceiling or wall but make sure wall colour is white or off white so you don't get a colour tint and faces look more natural and eyes don't squint.
3. Automatic exposure, if using TTL capable flash, pop-up flash is also TTL capable.
4. With wireless trigger can remove flash from hotshoe and position to best angle or position away from camera to get pleasing lighting outcome.
5. Compensate for backlighted subject or fill flash (over powering the sun).
6. The reality is you will buy more than one for more professional results when time's come.
7. Look more professional..just joking..haha.
These are some of the uses as your camera is not HSS capable (high speed sync).
PS I would suggest to cover pop-up flash with tissue paper or get a plastic diffuser cup or diy yourself to soften the light but that means reducing light output somewhat.Much better than naked direct flash.
Do read up on flash techniques...pretty challenging you will find.
http://www.scantips.com/lights/vk750.html
The said flash is decent for the price but research carefully before committing.