What is a namespace?
Namespaces are primarily used as a mechanism for defining custom fields (that'll end up as metafields) that won’t clash with other apps’ metafields like e.g. Shopify Reviews (using a namespace of "spr"). Using the default ‘accentuate’ namespace will ensure you are defining a separate set of metafields, but you can always override this on a field-by-field basis if you like.
It is also a great way to (technically) group sets of fields that naturally belong together. If you for example need to have two fields, "title" and "rating" for videos, you could opt for grouping them under the namespace "video" giving you two fields called "video.title" and "video.rating". Then if you need the same two field for an image, you could place these under the namespace "image" giving you "image.title" and "image.rating" that don't collide with the video fields.
If you need to use ACF to manage metafields created from other apps, you can just match the namespace and key for the fields when defining the field type and ACF will automatically use any existing values going forward. Just take care to define a field type that makes sense with regard to any existing data.