The maintenance and upgrades of tools, database systems, etc. and the underlying infrastucture is the responsibility of the PaaS Cloud Provider.
Various pricing models may allow paying only for what you use. This, for example, can allow an individual or a small organization to use sophisticated development software that they could not afford if it was installed on an internal, dedicated server.
Some PaaS Providers provide development options for multiple platforms: mobile, browser, and so on. If you or your organization want to develop software that can be accessed from multiple platforms, this might be an easy way to make that happen.
If you have events such as high seasonal sales activity, then the elasticity of the Cloud with PaaS might provide an opportunity.
The PaaS Cloud Provider may provide better security than your existing software (security—or inadequate security—can also be a disadvantage). Better security may come in part because it is critical for the PaaS Cloud Provider and is part of their main business. In-house security, on the other hand, is not usually an individual's or a organization's main business and, therefore, may not be as good as that offered by the PaaS Cloud Provider.
No need to manage the introduction of new releases of the development or underlying software. This is handled by the PaaS Cloud Provider.
No need to provision servers. This is handled by the PaaS Cloud Provider.
No need to manage the underlying data center. This is handled by the PaaS Cloud Provider.
Usually, there is no need to manage backups. This is handled by the PaaS Cloud Provider.
If the PaaS Cloud Provider supports failover should the software (for example, the database management software) or the data center become unavailable, that failover is a concern of the PaaS Cloud Provider and you do not need to plan for it.