What are the social and economic benefits of choosing to pay for an optional insurance policy (i.e. one that is not required by law) like fire insurance?
Very few, until you need that insurance policy.
Ireland is, explicitly, neutral and has no real intent to engage in warfare, though it engages in peacekeeping. That is an entirely valid position, but is also very different from the UK's which has committed to collective defense within NATO.
France's 1.9% is not all that different from 2.2% and France is among the more committed of NATO's big members (USA aside).
Germany is a member of NATO but largely, due to both historical reasons and political budgetary expediency, does not pull its weight to support the collective defense that it has committed to and relies on the efforts of others (or at least it did until Mr. Putin helpfully corrected its outlook). Canada is in a similar spot but is arguably less exposed to actual military risks itself, so less reliant on the efforts of others. Spain I am less familiar with, but is probably somewhat like Germany.
Other countries like the Baltics do see the risk in not paying up to the insurance kitty bank.
Now, you can argue that European countries should save their money and just take the risk. But, for Europe/NATO which is the context explicit in this question, is the ongoing Ukraine war in April 2022 the best moment to be making that argument?
p.s. Neutrality does not mean "no defense" either. While Switzerland rates a relatively low 0.8% GDP it also runs a conscription army, which is a not-inconsiderable imposition on the lives of its citizens.
I think the answer could be improved by expanding what those points are (relevant to economic and societal benefits)
You are typing and reading this on one such benefit. Literally.
what those points are (relevant to economic and societal benefits) and removing the discussion around the foreign policy / national security benefits of defense spending
Not really. There are very few benefits to military spending other than security national security. That's why I already stated.
Foreign influence can be achieved without military force. For all my respect for the Canadian armed forces, for examples in their battles in WW1, WW2 and Afghanistan (Passchendaele, Dieppe and Kandahar respectively), one could argue that a key item in Canadian international prestige would instead be its leadership in bringing the Ottawa treaty banning landmines to fruition. That did not need a Canadian military budget.
Once you are reasoning about other benefits than national security with regards to military budgets, you are losing the point. Yes, there can be benefits to having armed forces, but most of these could be realized by direct funding to the relevant subjects. University R&D for example, instead of military R&D. Civilian Search and Rescue capabilities for example, instead of using soldiers for catastrophe relief. Militaries are vastly too costly and too targeted to one particular role, that of inflicting deadly force if needed, for a well-run democratic nation to contemplate having them just for ancillary benefits (or for jobs-for-the-boys reasons).
So this whole notion about not talking about the benefits to national security, when talking about military expenditures is missing the point about military expenditures entirely. That's like asking about the benefits of owning a car, asides from driving it: cars are made for driving them.