Hi,
Apart from Sales, all other inventory related transactions are real-time and they will reflect immediately after transaction. As you mentioned, Sales is sent once in a day, the impact of sales will be visible after Sales IDoc posting. So Centralized inventory check is a default feature of SAP and nothing to be done for that.
POS DM also provides a facility of centralized place to analyze or see the transactional data. Assume you have 100 stores and you need to do Remote login to each and every store if you want to check something related to transactional data. If you have POSDM, all the transactional data is available in POSDM centrally. So you can have visibility on all transactional data at one place. It also have feature of trickle-feed wherein the transactional data is pushed to POSDM at a fixed interval of time. This allows you to collect the transactional data throughout the day and do processing at night. If you do collection and processing in one go, it will take time.
Usage of POS DM will increase the speed of posting only because of pushing data from all stores separately, you are pushing only from a centralized POSDM. POS DM helps you validate and check the transactional data before they are being pushed to SAP Retail. It also have powerful features of doing aggregation of various combinations as per requirement. It also has a features of rules which we can use to validate business data for certain prerequisites. For example, if due to network issue, same transaction have been received twice, POSDM should consider only 1 instance for processing and keep the duplicate in error. Apart from this, POSDM also have Analytics for various reports on POS transactional data.
In Google or SAP's website, you can look for POS DM features and advantages. Please see below link:
SAP POS DM (SAP Point-of-Sale Data Management) - SAP for Retail - SCN Wiki
Regards,
Amit