From a technical perspective, Odoo has a strong technical foundation, a modern architecture that is truly object-oriented and was evolved from the beginning by the community of open source. You can override almost ANY classes to modify the underlying layers. The odoo enterprise (paid version) itself is following the same rule. It means that it is nothing but a series of modules that transforms the Odoo core modules (free version) into something that not only looks totally different but it extends a lot of core functionality of the system to support complex scenarios. 

From the functional perspective, What a small or midsize business wants is not a complex system with hundreds of features claiming to support advanced workflows out of the box ! and takes years to be implemented! Most SMEs are looking for an ERP package that can quickly be implemented with functionalities to support the automation of their business process instead of making it more complicated. Odoo with a community of thousands of modules (Most of them are free) perfectly fits the bill.

From a business perspective, Odoo truly provides value to the organisation. Odoo provides all core customer relationship modules including Marketing, CRM, and Sales as well as supplier relationship modules like Purchase and warehouse. on top of it, there is a beautiful yet simple and sophisticated accounting module that works like a charm. 

Thanks to its open-source community, there is full transparency of core features and its development that just can't be found in any other competitor's development pipeline. 

SMEs are rapidly changing...  new integrations are needed to internal and external systems, Processes become more matured that require changes in the ERP flow and SMEs are don't want to ring the expensive consultants for all these changes. Odoo allows you to truly become independent by introducing Odoo Studio, A customisation tool that works inside the system and makes it easy to modify the system without the need for consultants.