To draw anything, is it necessary to draw its wireframe first?
The reason for drawing wire frames, or any construction method, is to properly develop the forms of the object. By laying out the underlying structure, you can get a better sense of volume, proportions and arrangement of the 3D object you're trying to map into a 2D art form. This lets you get the foundation ready before laying down time-consuming details that may be hard to fix later. 

If you focus first on the outline, you might recognize the plant as a plant, but you'll lose out on the understanding of the curved blades that make the leaves, or the cylindrical shape of the pot. 

Generally speaking, if you start from the edges, the following mistakes become common: 


Incorrect proportions, as the base layout wasn't developed first
Poor impression of depth, since outer lines become over emphasized compared to spatial relationships 
Too much time spent on making the outline "perfect" leads to slow progression and hyper focus on small areas and not the larger image


These problems can leave the drawing looking dimensionless and misshapen. 

Additionally, as you practice construction of real objects, you develop your natural understanding of how objects work. This allows you to become increasingly capable of constructing objects from imagination, or filling in blanks of objects you can't fully see. This is a skill that only comes about with practice.

Now, there are methods of practice and application where you focus on drawing the outline. One exercise is drawing the negative space around an object, effectively leaving you with a silhouette or outline. In such a case, you focus strongly on the relationship of angles and distances between empty spaces. Here, you're also training your eye to interpret 3D objects as 2D forms. 

This is also an essential skill. So to answer your question about whether it's always necessary to draw the wireframe first, I would say no. Specifically, learning to draw negative space is highly valuable, and can be combined with your understanding of construction to make believable objects.