Because in most situations you will find that SQLyog is more simple to use and more intuitive than the tools from MySQL. During the time that SQLyog has existed, MySQL have distributed at least 3 generations of GUI tools
* The MySQL 'Control Center'
* MySQL 'GUI tools' (including 'Administrator' and 'Query Browser')
* MySQL 'Workbench'
Workbench is the current generation. It does in principle 3 different things (as of 5.2.x version)
1) Manage server instances and users (this is migrated functionality from 'Administrator').
2) Modeling and diagramming. In older Workbench versions this was the only functionality.
3) Data manipulation including backup/restore (this is migrated functionality from 'Query Browser' except backup/restore what is migrated functionality from 'Administrator).
as regards 1)
We do not manage server instances in SQLyog and we have no intention to do it in the future either. We do have full GUI user management functionalities however.
as regards 2)
We don't do 'modelling' but we do 'diagramming' (in the Schema Designer feature). The SQLyog documentation explains the difference: "Whatever you add or change from the Schema Designer will be stored to the database exactly the same way as when you work with the menu's of the program. There is no need to 'upload', 'apply', 'synchronize' the LAYOUT or anything like that. SQLyog Schema Designer is a graphical implementation of direct client functionalities. We work with the database that we are connected to - not with any kind of 'model' internal for the program
(but note that if you want a 'model' before applying the changes to the 'real' or 'live' database you can simply use another database as a 'model' - and use Schema Synchronization to synchronise from the 'model database' to the 'real database'. The two databases need not be on the same server)."
as regards 3)
This we believe do much more efficiently then Workbench will ever be able to due to its architecture.
Add to this that SQLyog has lots of advanced features that Workbench has not: Schema and Data Sync Tools, 'Notfications services' (a database maintenance and reporting tool), GUI Query Builder, Query Profiler .. and more. See full feature list:
Also the SQL editor in SQLyog is more advanced (with autocompletion, "paranthesis'es matching" and "statement folding". SQLyog editor is very close to a full featured IDE (Eclipse, Visual Studio etc.) in this respect. This also makes a big difference when it comes to productivity. Further SQLyog has 'column width persistence' for table and results columns (across sessions) and a FORM view option. When SQLog opens a table or a result set for viewing or editing everything is made ready for to work with you at the very moment it opens. Multiple connections tabs and query tabs lets you organize you SQL statement exactly as you want it. 'favorites' gives fast access to SQL-statements and snippets used most frequently.
The various generations of GUI tools from MySQL have co-existed with SQLyog on users' desktop for years and Workbench does not change this. They do different things. There is some overlapping (data manipulation, user management and backup/restore), but here we believe that Workbench will never be able to compete with SQLyog in terms of productivity, efficiency and reliability.