Why, When, Why is it better?
Well, legally you don't. An invoice from the supplier should be enough for you to register the expense in accounting and for you to be able to use the photos. But how can you use the photos? Many people assume that once they have paid for the photos they can use them any way they want. This is not necessarily true.
Photos represent most of the time a creative work and therefore are subject to copyright laws. This is where it becomes complicated, especially nowadays where the global reach of content is just a click away. Not all countries have the same rules in respect to copyright, actually quite far from it.
As a rule of thumb (far from being all-inclusive), once a photographer produces a photo all copyrights and IP rights (Intellectual Property rights) belong to the photographer unless otherwise stated. The photographer will grant some or all rights of usage or even the IP rights (not so common) to the beneficiary of the photos. This is where a written agreement comes into place.
One might think that it is better to not have anything in writing but one would be very wrong. Let us assume that something goes wrong and you need to present your case in a court of law in front of a judge. The judge can only relate to whatever is in the written proven form: conversations, messages a written agreement (the best option). So rather than not including details in a written agreement is always better to include as much info and terms as possible.
What should be included in the written agreement?
Besides the general aspects (like: seller-buyer info, pricing, payments, Force Majeure, etc.) the agreement should include at least the following aspects:
What rights are granted by the photographer to the customer. It can be usage writes, IP rights, etc.
What rights are withheld by the photographer? In most cases, the photographer will keep the IP rights over the photos as well as the right to use the photos in his own portfolio or use them for educational purposes.
How long in time are the usage rights given for? The customer might get a perpetual license or a limited one (in terms of how long he is allowed to use the photos)
In which territory is the beneficiary allowed to use the photos: might be a city (very rare), a country, continent, or whatever is agreed.
In which form is the beneficiary allowed to use the photos? It can be in digital form, print form only, banners, etc.
Keep in mind the above info for future collaborations with photographers. Not all of them will have an agreement prepared, covering the above and this does not show some bad intentions or lack of knowledge from the photographer, just shows that he/she did not work before with advertising agencies or big customers.
There are also fields in photography where it is not common practice to have such a detailed agreement: wedding and baptism photography. This is mostly due to the fact that the photos do not have much value for resale or reuse by the photographer. Sometimes, such photos may be sold by the photographer on stock photo websites. So if you do not want to have your wedding photos in PowerPoint presentations maybe you should look into this. There is another reason why this reuse does not happen very often: to be able to resale your wedding photos the photographer should have a release model form signed by you and the guests. So I wouldn't worry too much about this!
Last piece of advice, when in doubt do not hesitate to consult a lawyer!
Stay safe & creative!