Reasons to return home and strong/complelling ties differ from country to country, city to city and individual to individual.
Some typical examples of ties can be a job, a house, social and family relationships, a bank account, possessions etc
The ECO will be looking to see if the aspects of life that bind someone to their home country outweigh those aspects that may cause them to be unwilling to return, or to attempt to change or extend their visa status.
That's it.
The cost is £83
The waiting time is just 2-3 weeks usually.
Nothing to lose and everything to gain.
In connection with the settlement route visa (spouse/fiancee) .....
1. do you already meet the Financial requirement of gross annual salary at £18,600 ?
2. are you sure you're ready for marriage ?
Technically, as long as you can demonstrate a Genuine and Subsisting Relationship in line with UKVI requirements and have met face-to face at least once then you'd be eligible.