Unboxed businesses have no boxes between the applications
Imagine the rail transport in a country - and all the tracks and trains are operated by different companies. Not only that - only few adhere to the same standards (i.e. the width of the track). Whenever you are travelling from one city to the next, and the tracks at some places are operated by a different company, the train cannot go further - all passengers with their luggage have to alight from the train, walk to the next train, and continue the journey. That is a very poor experience from the traveller perspective.
Take this analogy and apply this to your customer's data. When you transact with a customer (from lead acquisition to invoicing and post-sales support), your customer is on a journey with your company. Whenever the customer data needs to move between applications (for example, from your CRM to your accounting software), you'd want this to be smooth and uninterrupted. If you have applications that are not designed to work together in this process, you are building the equivalent of the different sizes of railroad tracks in your business - the data (all passengers) have to alight from one application (train) and enter the next in order to continue the journey.
The challenges with custom integrations
Integration is the answer - but what is integration? Back to the analogy of the trains - what if we have a way that the passengers and their luggage are transported from one train to the next (in absence of the "beam" technology as seen in "Star Trek" © CBS Studios Inc. - one could imagine the passengers and their luggage being carried to the next train) - the passengers will experience a different surrounding. The chairs are different, the interior decor is not what they are used to, the lavatories are at a different place, etc.
For your users in your business, they will notice that the business application has a different "look and feel" to it. The data does not have the same definitions either. The "Account" becomes the "Company", and the forms all look different. From a system administration perspective, user authorization (who can access the system) is handled differently, and authentication (once in the system, what can a user do) is also managed differently - certainly not from a central administration hub.
When you have created an integration (or paid someone for it), great! That should keep you going - well, you do need to monitor the integrations for performance, but let's say the mean time between failure is long enough for that not to be an issue. The issue is that any time one of your integrated applications has an update, you need to verify the integration point, and where needed take remedial action. For "spoke" applications (one integration point), that is very manageable. For "hub" applications, this can become a major headache very rapidly.
Monoliths or pebbles
A fully integrated business suite, or an ERP (Enterprise Resource Planning) sounds like a great solution. And companies like SAP, Oracle, and in lesser extent Microsoft, have made significant investments in functionality and technology. Each has their strengths and weaknesses, whether it be functionality richness, usability (compared to the other ERP systems) or simply price. They all have one thing in common - they are complex, as the integrated suite requires an integrated plan to implement. These cost can range from several hundred thousand dollars to well in the millions. And with that complexity comes a high interdependence amongst the modules - the ERP is like cement - you can shape it when it is wet - you are stuck with it when it has set.
Best of both worlds - enterprise grade applications without the enterprise grade headaches or prices
The best of an integrated suite without the complexities? AND the best of the individual point-solutions without the nightmare of the spaghetti-like integration? Here is how that is done: a suite of applications that you can license and use individually, but also are available in a bundle. For example - if you are only after a CRM application, you license the CRM application. When you add to that an accounting application (or helpdesk ticketing system, enterprise project management server, social and email marketing campaign software), then those systems snap together like matching pieces of a puzzle.
With the Zoho Suite of products, you have the choice of getting applications individually licensed, via bundles (such as CRM Plus), and as an "all-you-can-eat-buffet" with Zoho One.