There is no technical reason to make that decision. It depends on the business needs and the object itself.
There is no reason to add a feature that your users won't use -- it just adds to clutter for admins and users alike.
First, it is important to understand what an Activity is, and what it means to have activities. From the SOAP API documentation on Task and Event:
Task represents a business activity such as making a phone call or other to-do items.
Event represents an event in the calendar.
In the user interface, Task and Event records are collectively referred to as activities.
My personal definition is closer to this:
An Activity is something that someone did/will do on a date with one or more people about a thing.
An Event is an Activity that also has a duration and time of day.
A Task is an Activity that also has a status and maybe a disposition.
Some common examples of Tasks and Events are:
- Phone calls
- Relationship management to-do's (call Bob on Thursday about the Google deal)
"Having activities" means that a Task or Event can be "Related To" that type of object (that is, a thing in my definition above).
If your business will, e.g., track a meeting with an outside party about this particular thing, it is probably useful to allow activities. If not, it's just clutter for future admins.
So let's come up with some examples where activities don't make sense:
- I need to keep track of relationships between individuals (parent/child or friend/friend, etc). I can see a use case for tracking that, but I cannot imagine having a meeting to discuss John's relationship with Mike.
- I care about a person's historical addresses, so I track them in a custom object. That is also not really worth writing an email to anyone about.
- I need to keep track of job applications and their statuses. If it's just to track the status of the application, you wouldn't need activities. However, if I'm going to interview the person and keep notes in Salesforce, I may do that "related to" the application record. In that case, I would allow activities on that object.
Note - This decision is not final. You can turn those on or off as you wish.