This is my first result of working through a CGcookie tutorial. I thought that a grip could be a good idea for te crate.
The concept of separating parts of the mesh was new for me. At the end what do you do with the parts: parent, join or leave them as they are ?
Cool looking snowman man.
And yeah, think about having different objects or separating them like things are in real life. If a plastic chair is made of a single part, you try to model it as such, just so it doesn't have any obvious seams. And things that have different parts, like a lamp or PS5 controller, you have all the objects separated just because it wouldn't make any sense having the controller a single mesh. Plus it would be a nightmare. And of course as you say you can rig and animate the parts and stuff.
Nice models, Steve!
During modeling, it is often very handy to model parts as separate Objects (they can use different Modifiers, just to name 1 advantage).
When the model is finished, it is up to you, mostly. In the case of your snowman,for instance, I'd leave at least the hat as a separate (Parented) Object (so you can easily change its position and orientation on the head and Duplicate it if you want and so on), I'd Join the crate parts into 1 Object (unless you want to also show a version, where a part is lying next to the main crate) and in almost all cases, I'd also Join a keyboard into 1 Object (or maybe 2), even though all the keys are separate in real life (think of the 'nightmare' if you'd have to name each key uniquely).
But you'd have to consider each situation individually. When in doubt, it might be better to leave them as separate Objects; it is always easier to Join them later on, then to Separate Parts afterwards.