As others have said, when it's countable it's more specific, when it's uncountable it's more general. The main difference between using countable or uncountable is the way you think about the information.
"The department is capable of providing specialized rescue service."
Because "service" is uncountable here, it refers to the service in a general sense. In this sentence we can think of "service" as a term that encompasses every different thing that can be done by the department when they do a specialized rescue. Maybe in different situations they do different things, so the general "service" refers to all those possibilities.
If we make it countable...
"The department is capable of providing a specialized rescue service."
This suggests that the department offers many types of service, but we're specifically talking about the service of specialized rescue. The sense I get from this sentence is that the specialized rescue service is one option among others.
Let's look at your other example.
"The bank is offering a new service that will allow patrons to examine personal transactions listed in a consolidated weekly statement."
"Service" is countable here because it specifies one type of service, namely the ability to examine personal transactions in a weekly statement. The bank may offer other types of service too, but in this sentence we're only talking about weekly statements.
If we make it non-countable...
"The bank is offering new service that will allow patrons to examine personal transactions listed in a consolidated weekly statement."
This sentence sounds awkward to me. It could be correct if the weekly transaction statement is only one part of the new service that the bank offers. However, because the sentence discusses a very specific description of the new service, making it countable sounds more logical.
I hope that makes sense. The difference is subtle, so it's hard to explain well.