Deinterlacing and interlacing are used primarily to aid in the rotoscoping of field-based material, or in the application of filters to field-based material. Deinterlacing and interlacing are not necessary for processes that support field-based rendering.
Because there is a line-based difference in time per frame of interlaced video, if you paint or filter field-based frames, artefacts can be introduced. For example, if you blur over fields, colour information from pixels on the F1 field is smudged into the pixels on the F2 field.
Deinterlacing clips prior to painting or filtering separates each field, allowing you to work on separate fields and achieve better results. Once the effects have been added, re-interlace the clip.
