Many businesses use heavily customized SharePoint Servers. It is only after the migration is complete that they realize not all workloads can be replicated with high fidelity on SharePoint Online as the architecture of the services are fundamentally different. While a hybrid implementation can solve some of these problems, it is often complex and too labor-intensive even for bigger businesses with established IT teams to handle the process by themselves.
Here are some scenarios in which businesses migrate from SharePoint 2013 to SharePoint Online
Some businesses continue to use Microsoft Access Services that is not available in SharePoint Online.
Many users require the SharePoint Foundation edition, last available in SharePoint 2013, that did not require user licensing costs, unlike SharePoint Online that is available only on a per-user subscription basis.
Businesses need to create and maintain public-facing websites that cannot be supported with SharePoint Online since the SharePoint Online Public Websites feature was retired.
Businesses need extensive customization and administration options. Not all SharePoint Online plans have all the features that businesses might need. Some of the features built into the on-premises servers require separate licensing, subscriptions, or third-party application integration in SharePoint Online.
With access to SharePoint Central Administration, users can modify the file type extensions and different thresholds that cannot be changed in SharePoint Online. Learn how to migrate SharePoint 2013 to 2019 in detail.
I hope this information will be helpful!