I would consider ‘someone who I wouldn’t confide in or show great vulnerability to but would talk about mutual interests and/or would enjoy hanging out with’ to be a casual friend.
My ideal set of relationship patterns includes a wide variety of casual relationships. What do I think causal relationships offer that I am (at least less likely) to get from closer relationships?
1) Mutual interests not shared with other relationships. I have never met anybody who shares every single one of my interests, and I doubt I ever will. When people in close relationships don’t share a particular interest of mine, my casual friends are there when I really want to explore it in good company.
2) Relaxation. Casual relationships and close relationships get me to relax in different ways. In close relationships, I relax in the sense that I am secure that I will be valued unconditionally. However, close relationships carry a higher level of responsibility than casual relationships. The lack of responsibility in casual relationships can be a relief.
3) Learning. Generally, we form close relationships with people who share similar ideas and views of the world, and who have a similar set of knowledge. There are some exceptions, but generally if I want to be exposed to new ideas or get access to a wider set of knowledge, casual relationships do this way better than close relationships.
Here is a prime example of the value of casual relationships: my relationship with the participants of the ace-spectrum community. I have never been in a close relationship with anybody openly on the ace-spectrum. However, when I really want to get into conversations about the ace-spectrum which goes beyond asex/aro 101, ace-spectrum spaces are there to host them. I have also LEARNED A TON from other people on the ace-spectrum.
And I think casual relationships improve the quality of close relationships. For example, my dad is very interested in Perry Mason. However, he is not in a close relationship with anybody who wants to listen to what he has to say about Perry Mason. This means a) we either have to listen to him talk about Perry Mason, in spite of our lack of interest b) my dad has to keep silent about something he really wants to express or c) he can go to an online Perry Mason discussion group. Clearly (c) is the best course of action.
And I the extreme-nuclear-family model (marriage+children above all, which even excludes even the parents of the married couple) is unstable not only because it discourages forming other kinds of close relationships, it even discourages investing in more casual relationships. Yes, casual relationships require less investment per relationship, but the required investment is still greater than zero.
Of course, close relationships get priority over casual relationships. That’s one of the things which distinguishes them. However, having a diverse set of relationships, both casual and close, is best. The way I look at it, close relationships offer depth, and casual relationships offer breadth.
To the extent possible under law,
the person who associated CC0
with this work has waived all copyright and related or neighboring
rights to this work.
The Notes Which Do Not Fit 
I can relate to the “mutual interests not shared” point a lot. Maybe I’m an extreme case, but none of my significant others or close friends shares any major interests of mine. My connections with them are all based on shared values. Some also share a bit of my interests (e.g. a particular book or musical), which is already a bonus. As a poly, I know too well that it’s impossible to expect one single person to fulfill all my needs.
I don’t think anybody, poly or not, should expect any one person to fulfil of their needs (I am assuming you mean ‘needs’ in a broad sense).
That’s very true. But the mainstream mindset is one’s romantic partner should fulfill their every need, which has caused a lot of frustration and drama in relationships.