Truth is immutable. It is ever constant. It cannot grow, neither can it perish. It cannot shines, neither can it fades. Take a fist full of soil and observe. Can it remain same after 15 days? It cannot. May be, you cannot observe with naked eyes but small organisms would grow or die inside it resulting in changes.
Thats where Shiva comes to define the immutable truth. He smears His body with ashes, which doesn't have seeds to allow flourishing the life or even death. He is ever constant. He never takes birth nor dies neither takes Avatar. In the world full of dynamism there must be something against which dynamism can be observed. The parameters may be both velocity and time.
So with ever changing growing and perishing Srishti and beyond, Vishnu changes Himself every moment. He is not restricted to only Avatars. He becomes everything whatever you can imagine or whatever not. Every change is driven by Maya. Whatever is seen and whatever is not is driven by Maya and is Param illusion. Vishnu is Maya Pati. He is making all that happen and interestingly He is fully involved in that. Maya Pati Himself is part of Maya.
Humans don't have and won't ever have capabilities to understand the truth penetrating the Maya Jaal. And that's where the term Bhav Sagar comes and Yogis keep trying to sail through it and feel the real truth.
Now the reader might ask 'so what' or 'whats the point' or 'arre bhai kahna kya chahte ho'. So, the entire system is a 'Chakki' and a stone wheel is moving over a stationary stone wheel rotating with respect to a stationary hinge. Input is wheat and output is flour. Chakki processes it to nullify its existence completely. It appears that only moving stone is doing its job and stationary is not doing anything but the entire system is equally important.