Hi,
I want to understand why the customer journey starts with contacts? A contact is someone who is already a part of your system.
There are there types of the contact who can be set as the audience of the customer journey.

Not all among them are already a part of the system. For example, “Submitted a form” stands that the contacts who submit a certain form, which may be used to update existing contacts or create non-existent contacts according to the Matching Strategy you have set.
And why not lead? Or any anonymous person?
The audience can be contact only because the above three types can only filter out the records of Contact.
However, the following steps in the customer journey can be targeted at Contact or Account. For more details, please refer to: Guide your prospects through an interactive customer journey (Dynamics 365 Marketing) | Microsoft Docs.
What is the point of creating a marketing journey where your Target is always the same people?
It can provide you with a more convenient way of handling some fixed processes. Of course, they are definitely not always “the same people” for the contacts in the above source types will change. For example, you may need to send regular emails to subscribers who have subscribed to a particular topic. Subscribers will add and unsubscribe. With Customer Journey, you don't need to adapt your process to these changes.
And after sending an email to the target segment of contacts, containing a marketing form, what is the point of creating a lead/contact when that particular contact is only going to submit the form?
Depending on the type of marketing form you create, how you update it and the Matching Strategy, it is possible to compare the data submitted by contacts after they have submitted the form and decide whether to create or update contacts or leads. For more details, please refer to: Set up and manage marketing forms for use in marketing pages (Dynamics 365 Marketing) | Microsoft Docs
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya