I concur with what others have posted, and will just expand a bit on the surface disturbance because this is crucial in planted tanks.
You may or may not need a filter for filtration in a planted tank, depending upon the fish and plants (species and numbers of both). But surface disturbance is important, and especially so at night. It is possible to poison fish from CO2 during the dark (night) period.
Fish, plants and some bacteria respire 24/7, taking in oxygen and expelling CO2. CO2 is also produced in the bacterial breakdown of organics, primarily in the substrate. The CO2 from this latter source is almost always considerably higher than the respiration, again depending upon the specifics. All of this is on-going 24/7.
The plants during photosynthesis continue to repire but they also take up CO2 for their carbon needs. The light intensity drives photosynthesis, but it cannot continue if CO2 (and the other essential nutrients) are not sufficient. With fast-growing plants, and depending upon the light intensity and duration, it is common for all of the available CO2 to be utilized by the plants. During photosynthesis, oxygen is produced and released by the plants as a by-product. During the daylight (under the tank lighting) this should cause no issues, but at night it can.
The uptake of CO2 by plants is non-existent during the night, and CO2 can build up. This is what can cause trouble for the fish; the CO2 level increases and oxygen depletes. Maintaining a good amount of surface disturbance especially at night will allow the CO2 to partially out-gas, and more oxygen will enter the water.
I had evidence of this a few years ago in my 70g tank. One morning just after the lights came on I noticed the Corydoras were respirating faster than normal, and surface breaching more. I increased the surface disturbance by adjusting the filter return so it rippled the water surface much more at the one end. Problem solved; from then on, the cories in the early morning are respirating normally, and rarely surface breach.
You do not need a raging whirlpool, but some good surface disturbance to maintain an exchange of oxygen and CO2 at the surface. Air stones do not achieve this, it has to be surface disturbance. The filter is the usual means of achieving this, so this is the primary need for a filter. Water circulation is also improved obviously.