Too many hard inquiries on a person's credit file will show the person's been desperately shopping around for credit they probably don't qualify for. That will make other potential lenders more wary.
Technically, too many hard inquiries on a credit file will also reduce the FICO score but only by a small amount, and only for the first year. However, they will stay on your file for at least 3 years with Equifax and 6 years with TransUnion. The chief harm done will be the first reason. If a mortgagee sees you've made 10 other applications in the past month, they'll look at you a lot more pessimistically. A credit score is frequently only one of the factors lenders use in assessment.
Hard inquiries were originally created for the purpose of discouraging consumers from shopping for too much credit. Not a bad idea. However, due to lax enforcement, as often as not, they are used by collection agencies and debt buyers as an enforcement tool to collect on bad debts.
Soft inquiries are used for account updates and inquiries by creditors and collection agencies. They are also used in connection with insurance underwriting, tenancy and employment applications. These inquiries are generally in a more summary form and contain only as much information as the client is willing to pay the bureau for. They may also feature additional software like "Gemini" to yield a fraud score to potential creditors. Again, all information is available to almost anyone. It just boils down to how much they're willing to pay. Irregarless of their form, soft inquiries aren't displayed to anyone except the requestor and the subject of the report. Thus they have no effect on your score or profile with other lenders.
It should also be noted that almost any government agency can pull your credit file as part of an investigation, and it will not appear under either category.